Semiconductor device

ABSTRACT

Provided is a highly integrated semiconductor device, a semiconductor device with large storage capacity with respect to an area occupied by a capacitor, a semiconductor device capable of high-speed writing, a semiconductor device capable of high-speed reading, a semiconductor device with low power consumption, or a highly reliable semiconductor device. The semiconductor device includes a first transistor, a second transistor, and a capacitor. A conductor penetrates and connects the first transistor, the capacitor, and the second transistor. An insulator is provided on a side surface of the conductor that penetrates the capacitor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. Furthermore, the present invention relates to a process, amachine, manufacture, or a composition of matter. In particular, thepresent invention relates to, for example, a semiconductor, asemiconductor device, a display device, a light-emitting device, alighting device, a power storage device, a memory device, or aprocessor. The present invention relates to a method for manufacturing asemiconductor, a semiconductor device, a display device, alight-emitting device, a lighting device, a power storage device, amemory device, or a processor. The present invention relates to a methodfor driving a semiconductor device, a display device, a light-emittingdevice, a lighting device, a power storage device, a memory device, or aprocessor.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A display device, a light-emitting device, a lightingdevice, an electro-optical device, a semiconductor circuit, and anelectronic device include a semiconductor device in some cases.

2. Description of the Related Art

A technique for forming a transistor by using a semiconductor over asubstrate having an insulating surface has attracted attention. Thetransistor is applied to a wide range of semiconductor devices such asan integrated circuit and a display device. Silicon is known as asemiconductor applicable to a transistor.

Whether amorphous silicon, polycrystalline silicon, single crystalsilicon, or the like is used as a semiconductor in a transistor dependson the purpose. For example, in the case of a transistor included in alarge display device, amorphous silicon, which can be formed using anestablished technique for forming a film over a large-sized substrate,is preferably used. On the other hand, in the case of a transistorincluded in a high-performance display device where a display device anda driver circuit are formed over the same substrate, polycrystallinesilicon, which can form a transistor having high field-effect mobility,is preferably used. In the case of using a transistor included in anintegrated circuit or the like, it is preferable to use single crystalsilicon having higher field-effect mobility. As a method for formingpolycrystalline silicon, high-temperature heat treatment or laser lighttreatment which is performed on amorphous silicon has been known.

In recent years, an oxide semiconductor has attracted attention. Anoxide semiconductor can be formed by a sputtering method or the like,and thus can be used for a semiconductor of a transistor in alarge-sized display device. A transistor including an oxidesemiconductor has high field-effect mobility; therefore, ahigh-performance display device where a display device and a drivercircuit are formed over the same substrate can be obtained. In addition,there is an advantage that capital investment can be reduced becausepart of production equipment for a transistor including amorphoussilicon can be retrofitted and utilized.

As a method for providing a transistor including an oxide semiconductorwith stable electrical characteristics, a technique where an insulatorin contact with an oxide semiconductor is doped with oxygen is disclosed(see Patent Document 1). The technique disclosed in Patent Document 1enables oxygen vacancies in an oxide semiconductor to be reduced. As aresult, variation in electrical characteristics of a transistorincluding an oxide semiconductor can be reduced and reliability can beimproved.

A transistor including an oxide semiconductor is known to have anextremely low leakage current in an off state. For example, a low-powerCPU and the like utilizing a characteristic of low leakage current of atransistor including an oxide semiconductor are disclosed (see PatentDocument 2).

Patent Document 3 discloses that a transistor having high field-effectmobility can be obtained by a well potential formed using an activelayer formed of a semiconductor.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2011-243974-   [Patent Document 2] Japanese Published Patent Application No.    2012-257187-   [Patent Document 3] Japanese Published Patent Application No.    2012-59860

SUMMARY OF THE INVENTION

An object is to provide a highly integrated semiconductor device.Another object is to provide a semiconductor device with large storagecapacity with respect to an area occupied by a capacitor. Another objectis to provide a semiconductor device capable of high-speed writing.Another object is to provide a semiconductor device capable ofhigh-speed reading. Another object is to provide a semiconductor devicewith low power consumption. Another object is to provide a highlyreliable semiconductor device. Another object is to provide a novelsemiconductor device.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

(1) One embodiment of the present invention is a semiconductor deviceincluding a first transistor, a second transistor, a capacitor, a firstinsulator, and a second insulator. The first transistor includes a firstsemiconductor, a third insulator, and a first conductor. The secondtransistor includes a second semiconductor, a fourth insulator, and asecond conductor. The capacitor includes a third conductor, a fourthconductor, and a fifth insulator. The first insulator is disposed overthe first transistor. The capacitor is disposed over the firstinsulator. The second insulator is disposed over the capacitor. Thesecond transistor is disposed over the second insulator. The firstsemiconductor includes a region overlapping with the first conductorwith the third insulator interposed therebetween. The secondsemiconductor includes a region overlapping with the second conductorwith the fourth insulator interposed therebetween. The third conductorincludes a region facing the fourth conductor with the fifth insulatorinterposed therebetween. The third conductor includes a region incontact with the first conductor through an opening in the firstinsulator. The third conductor includes a region in contact with thesecond semiconductor through an opening in the second insulator.

(2) One embodiment of the present invention is the semiconductor devicedescribed in (1), in which the fifth insulator includes an elementincluded in the fourth conductor.

(3) One embodiment of the present invention is the semiconductor devicedescribed in (1) or (2), in which the first semiconductor includessilicon.

(4) One embodiment of the present invention is the semiconductor devicedescribed in any one of (1) to (3), in which the second semiconductorincludes indium.

A highly integrated semiconductor device can be provided. Asemiconductor device with large storage capacity with respect to an areaoccupied by a capacitor can be provided. A semiconductor device capableof high-speed writing can be provided. A semiconductor device capable ofhigh-speed reading can be provided. A semiconductor device with lowpower consumption can be provided. A highly reliable semiconductordevice can be provided. A novel semiconductor device can be provided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention.

FIG. 2 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention.

FIG. 3 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention.

FIG. 4 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention.

FIGS. 5A to 5D are Cs-corrected high-resolution TEM images of a crosssection of a CAAC-OS and a cross-sectional schematic view of a CAAC-OS.

FIGS. 6A to 6D are Cs-corrected high-resolution TEM images of a plane ofa CAAC-OS.

FIGS. 7A to 7C show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD.

FIGS. 8A and 8B show electron diffraction patterns of a CAAC-OS.

FIG. 9 shows a change in crystal part of an In—Ga—Zn oxide induced byelectron irradiation.

FIGS. 10A to 10C are a cross-sectional view of stacked semiconductorlayers and band diagrams thereof.

FIGS. 11A to 11C are cross-sectional views illustrating a method ofmanufacturing a transistor of one embodiment of the present invention.

FIGS. 12A and 12B are cross-sectional views illustrating a method ofmanufacturing a transistor of one embodiment of the present invention.

FIG. 13 is a cross-sectional view illustrating a method of manufacturinga transistor of one embodiment of the present invention.

FIG. 14 is a cross-sectional view illustrating a method of manufacturinga transistor of one embodiment of the present invention.

FIG. 15 is a cross-sectional view illustrating a method of manufacturinga transistor of one embodiment of the present invention.

FIG. 16 is a cross-sectional view illustrating a method of manufacturinga transistor of one embodiment of the present invention.

FIG. 17 is a cross-sectional view illustrating a method of manufacturinga transistor of one embodiment of the present invention.

FIGS. 18A and 18B are each a circuit diagram of a semiconductor deviceof one embodiment of the present invention.

FIGS. 19A and 19B are each a circuit diagram of a memory device of oneembodiment of the present invention.

FIG. 20 is a block diagram of an RFIC tag of one embodiment of thepresent invention.

FIGS. 21A to 21F illustrate application examples of an RFIC tag of oneembodiment of the present invention.

FIG. 22 is a block diagram illustrating a CPU of one embodiment of thepresent invention.

FIG. 23 is a circuit diagram of a memory element of one embodiment ofthe present invention.

FIGS. 24A to 24C are a top view and circuit diagrams of a display deviceof one embodiment of the present invention.

FIG. 25 illustrates a display module of one embodiment of the presentinvention.

FIGS. 26A to 26F illustrate electronic devices of one embodiment of thepresent invention.

FIGS. 27A1, 27A2, 27A3, 27B1, 27B2, 27C1, and 27C2 illustrate electronicdevices of one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings. However, the present invention isnot limited to the description below, and it is easily understood bythose skilled in the art that modes and details disclosed herein can bemodified in various ways. Furthermore, the present invention is notconstrued as being limited to description of the embodiments. Indescribing structures of the present invention with reference to thedrawings, common reference numerals are used for the same portions indifferent drawings. Note that the same hatched pattern is applied tosimilar parts, and the similar parts are not especially denoted byreference numerals in some cases.

Note that the size, the thickness of films (layers), or regions indiagrams may be exaggerated for clarity.

A voltage usually refers to a potential difference between a givenpotential and a reference potential (e.g., a source potential or aground potential (GND)). A voltage can be referred to as a potential andvice versa.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for the sake of convenience and do not denote theorder of steps or the stacking order of layers. Therefore, for example,the term “first” can be replaced with the term “second”, “third”, or thelike as appropriate. In addition, the ordinal numbers in thisspecification and the like are not necessarily the same as the ordinalnumbers used to specify one embodiment of the present invention.

Note that a “semiconductor” includes characteristics of an “insulator”in some cases when the conductivity is sufficiently low, for example.Furthermore, a “semiconductor” and an “insulator” cannot be strictlydistinguished from each other in some cases because a border between the“semiconductor” and the “insulator” is not clear. Accordingly, a“semiconductor” in this specification can be called an “insulator” insome cases. Similarly, an “insulator” in this specification can becalled a “semiconductor” in some cases.

Furthermore, a “semiconductor” includes characteristics of a “conductor”in some cases when the conductivity is sufficiently high, for example.Furthermore, a “semiconductor” and a “conductor” cannot be strictlydistinguished from each other in some cases because a border between the“semiconductor” and the “conductor” is not clear. Accordingly, a“semiconductor” in this specification can be called a “conductor” insome cases. Similarly, a “conductor” in this specification can be calleda “semiconductor” in some cases.

Note that an impurity in a semiconductor refers to, for example,elements other than the main components of a semiconductor. For example,an element with a concentration of lower than 0.1 atomic % is animpurity. When an impurity is contained, the density of states (DOS) maybe formed in a semiconductor, the carrier mobility may be decreased, orthe crystallinity may be decreased, for example. When the semiconductoris an oxide semiconductor, examples of an impurity which changes thecharacteristics of the semiconductor include Group 1 elements, Group 2elements, Group 14 elements, Group 15 elements, and transition metalsother than the main components; specifically, there are hydrogen(including water), lithium, sodium, silicon, boron, phosphorus, carbon,and nitrogen, for example. When the semiconductor is an oxidesemiconductor, oxygen vacancies may be formed by entry of impuritiessuch as hydrogen, for example. Furthermore, when the semiconductor issilicon, examples of an impurity which changes the characteristics ofthe semiconductor include oxygen, Group 1 elements except hydrogen,Group 2 elements, Group 13 elements, and Group 15 elements.

Note that in the embodiments described below, an insulator may be formedto have, for example, a single-layer structure or a stacked-layerstructure including an insulator containing one or more of boron,carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon,phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium,lanthanum, neodymium, hafnium, and tantalum unless otherwise specified.A resin may be used as the insulator. For example, a resin containingpolyimide, polyamide, acrylic, silicone, or the like may be used. Theuse of a resin does not need planarization treatment performed on a topsurface of the insulator in some cases. By using a resin, a thick filmcan be formed in a short time; thus, the productivity can be increased.The insulator may be preferably formed to have a single-layer structureor a stacked-layer structure including an insulator containing aluminumoxide, silicon nitride oxide, silicon nitride, gallium oxide, yttriumoxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide,or tantalum oxide.

Furthermore, in the embodiments described below, a conductor may beformed to have, for example, a single-layer structure or a stacked-layerstructure including a conductor containing one or more of boron,nitrogen, oxygen, fluorine, silicon, phosphorus, aluminum, titanium,chromium, manganese, cobalt, nickel, copper, zinc, gallium, yttrium,zirconium, molybdenum, ruthenium, silver, indium, tin, tantalum, andtungsten unless otherwise specified. Alternatively, a film of an alloyor compound containing the above element may be used: a conductorcontaining aluminum, a conductor containing copper and titanium, aconductor containing copper and manganese, a conductor containingindium, tin, and oxygen, a conductor containing titanium and nitrogen,or the like may be used.

In this specification, the phrase “A has a region with a concentrationB” includes, for example, “the concentration of the entire region in aregion of A in the depth direction is B”, “the average concentration ina region of A in the depth direction is B”, “the median value of aconcentration in a region of A in the depth direction is B”, “themaximum value of a concentration in a region of A in the depth directionis B”, “the minimum value of a concentration in a region of A in thedepth direction is B”, “a convergence value of a concentration in aregion of A in the depth direction is B”, and “a concentration in aregion of A in which a probable value is obtained in measurement is B”.

In this specification, the phrase “A has a region with a size B, alength B, a thickness B, a width B, or a distance B” includes, forexample, “the size, the length, the thickness, the width, or thedistance of the entire region in a region of A is B”, “the average valueof the size, the length, the thickness, the width, or the distance of aregion of A is B”, “the median value of the size, the length, thethickness, the width, or the distance of a region of A is B”, “themaximum value of the size, the length, the thickness, the width, or thedistance of a region of A is B”, “the minimum value of the size, thelength, the thickness, the width, or the distance of a region of A isB”, “a convergence value of the size, the length, the thickness, thewidth, or the distance of a region of A is B”, and “the size, thelength, the thickness, the width, or the distance of a region of A inwhich a probable value is obtained in measurement is B”.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.The term “substantially parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −30° and lessthan or equal to 30°. The term “perpendicular” indicates that the angleformed between two straight lines is greater than or equal to 80° andless than or equal to 100°, and accordingly includes the case where theangle is greater than or equal to 85° and less than or equal to 95°. Theterm “substantially perpendicular” indicates that the angle formedbetween two straight lines is greater than or equal to 60° and less thanor equal to 120°.

<Structure of Semiconductor Device>

FIG. 1 is a cross-sectional view of a semiconductor device of oneembodiment of the present invention. The left view in FIG. 1 illustratescross sections of a transistor 491 and a transistor 492 in the channellength direction. The right view in FIG. 1 illustrates a cross sectiontaken along the dashed-dotted line (a-a′) in the left view in FIG. 1.That is, the right view is a cross-sectional view, which is across thecenter of the conductor 404, in the channel width direction of thetransistor 492.

A semiconductor device illustrated in FIG. 1 includes the transistor491; an insulator 478, a conductor 469 a, and a conductor 469 b over thetransistor 491; a conductor 474 a over the insulator 478 and in contactwith the conductor 469 a; a conductor 474 b over the insulator 478 andin contact with the conductor 469 b; an insulator 480 over the insulator478, the conductor 474 a, and the conductor 474 b; an insulator 482 anda capacitor 493 over the insulator 480; an insulator 484, a conductor413, and a conductor 415 over the insulator 482 and the capacitor 493;an insulator 401 over the insulator 484, the conductor 413, and theconductor 415; an insulator 402 over the insulator 401; a transistor 492over the insulator 402; an insulator 408 over the transistor 492; aninsulator 418 over the insulator 408; and a conductor 424 a and aconductor 424 b over the insulator 418. The insulator 401 has a functionof blocking oxygen and hydrogen. Note that the conductor 413, theinsulator 401, and the insulator 402 may be included in the transistor492.

FIG. 3 illustrates an example of the semiconductor device of oneembodiment of the present invention, which is different from the examplein FIG. 1. A semiconductor device illustrated in FIG. 3 includes thetransistor 491; the insulator 478, the conductor 469 a, and theconductor 469 b over the transistor 491; the conductor 474 a over theinsulator 478 and in contact with the conductor 469 a; the conductor 474b over the insulator 478 and in contact with the conductor 469 b; theinsulator 480 over the insulator 478, the conductor 474 a, and theconductor 474 b; the insulator 484, the conductor 413, and the conductor415 over the insulator 480; the insulator 401 over the insulator 484,the conductor 413, and the conductor 415; the insulator 402 over theinsulator 401; the transistor 492 over the insulator 402; the insulator408 over the transistor 492; the insulator 418 over the insulator 408;an insulator 486 and a capacitor 494 over the insulator 418; and theconductor 424 a and the conductor 424 b over the insulator 486 and thecapacitor 494. Note that the insulator 401 has a function of blockingoxygen and hydrogen. Note that the conductor 413, the insulator 401, andthe insulator 402 may be included in the transistor 492.

FIG. 4 illustrates an example of the semiconductor device of oneembodiment of the present invention, which is different from theexamples in FIG. 1 and FIG. 3. A semiconductor device illustrated inFIG. 4 includes the transistor 491; the insulator 478, the conductor 469a, and the conductor 469 b over the transistor 491; the conductor 474 aover the insulator 478 and in contact with the conductor 469 a; theconductor 474 b over the insulator 478 and in contact with the conductor469 b; the insulator 480 over the insulator 478, the conductor 474 a,and the conductor 474 b; the insulator 482 and the capacitor 493 overthe insulator 480; the insulator 484, the conductor 413, and theconductor 415 over the insulator 482 and the capacitor 493; theinsulator 401 over the insulator 484, the conductor 413, and theconductor 415; an insulator 402 over the insulator 401; the transistor492 over the insulator 402; the insulator 408 over the transistor 492;the insulator 418 over the insulator 408; the insulator 486 and thecapacitor 494 over the insulator 418; and the conductor 424 a and theconductor 424 b over the insulator 486 and the capacitor 494. Theinsulator 401 has a function of blocking oxygen and hydrogen. Note thatthe conductor 413, the insulator 401, and the insulator 402 may beincluded in the transistor 492.

The semiconductor device illustrated in FIG. 1 is described below.Description on the semiconductor device illustrated in FIG. 1 can beapplied to the semiconductor devices illustrated in FIG. 3 and FIG. 4.The transistor 491 includes a semiconductor substrate 400; an insulator462 over the semiconductor substrate 400; a conductor 454 over theinsulator 462; and an insulator 470 in contact with a side surface ofthe conductor 454. The semiconductor substrate 400 includes a region 446a, a region 446 b, a region 444 a, and a region 444 b. The region 446 aand the region 446 b are regions in the semiconductor substrate 400which overlap with neither the conductor 454 nor the insulator 470. Theregion 444 a and the region 444 b are regions in the semiconductorsubstrate 400 which overlap with the insulator 470.

For the semiconductor substrate 400, a single-material semiconductor ofsilicon, germanium, or the like or a compound semiconductor of siliconcarbide, silicon germanium, gallium arsenide, gallium nitride, indiumphosphide, zinc oxide, gallium oxide, or the like may be used, forexample. For the semiconductor substrate 400, an amorphous semiconductoror a crystalline semiconductor may be used, and examples of acrystalline semiconductor include a single crystal semiconductor, apolycrystalline semiconductor, and a microcrystalline semiconductor.

The insulator 462 serves as a gate insulator of the transistor 491. Theconductor 454 serves as a gate electrode of the transistor 491. Theinsulator 470 serves as a sidewall insulator (also referred to as asidewall) of the conductor 454. The regions 446 a and 446 b serves as asource region and a drain region of the transistor 491. The regions 444a and 444 b serve as lightly doped drain (LDD) regions of the transistor491.

The regions 444 a and 444 b can be formed by adding an impurity usingthe conductor 454 as a mask. After that, the insulator 470 is formed andan impurity is added using the conductor 454 and the insulator 470 asmasks, so that the regions 446 a and 446 b can be formed. Thus, when theregions 444 a and 444 b and the regions 446 a and 446 b are formed usingthe same kind of impurities, the regions 444 a and 444 b have a lowerimpurity concentration than the regions 446 a and 446 b.

When the transistor 491 includes the regions 444 a and 444 b, ashort-channel effect can be suppressed. Therefore, such a structure issuitable for miniaturization.

The transistor 491 is kept away from another transistor provided in thesemiconductor substrate 400 by an insulator 442 or the like. AlthoughFIG. 1 shows an example where the insulator 442 is formed by a shallowtrench isolation (STI) method, one embodiment of the present inventionis not limited thereto. For example, instead of the insulator 442, aninsulator formed by a local oxidation of silicon (LOCOS) method may beused so that transistors are separated from each other.

Note that the structure of the transistor 491 is not limited to thestructure illustrated in FIG. 1. For example, a structure where thesemiconductor substrate 400 has a projection (also referred to as aprotrusion or a fin), like the transistor 491 illustrated in FIG. 2, maybe used. In the structure of the transistor 491 illustrated in FIG. 2,an effective channel width with respect to the occupation area can beincreased as compared with that illustrated in FIG. 1. Thus, theon-state currents of the transistor 491 can be increased.

The transistor 492 illustrated in FIG. 1 includes a semiconductor 406 aover the insulator 402; a semiconductor 406 b over the semiconductor 406a; a conductor 416 a and a conductor 416 b in contact with a top surfaceof the semiconductor 406 b; a semiconductor 406 c in contact with theside surfaces of the semiconductor 406 a, the top surface and sidesurfaces of the semiconductor 406 b, a top surface and side surfaces ofthe conductor 416 a, and a top surface and side surfaces of theconductor 416 b; an insulator 412 over the semiconductor 406 c; and aconductor 404 over the insulator 412. Although the conductor 413, theinsulator 401, and the insulator 402 are components independent of thetransistor 492 here, one embodiment of the present invention is notlimited thereto. For example, the conductor 413, the insulator 401, andthe insulator 402 may be included in the transistor 492.

When the conductor 413, the insulator 401, and the insulator 402 areincluded in the transistor 492, the conductor 413 serves as a gateelectrode of the transistor 492. The insulator 402 serves as a gateinsulator of the transistor 492. The conductor 416 a and the conductor416 b serve as a source electrode and a drain electrode of thetransistor 492. The insulator 412 serves as a gate insulator of thetransistor 492. The conductor 404 serves as a gate electrode of thetransistor 492.

The conductor 413 and the conductor 404 serve as gate electrodes of thetransistor 492, and may be supplied with different potentials. Forexample, by applying a negative or positive gate voltage to theconductor 413, the threshold voltage of the transistor 492 may becontrolled. By the conductor 413, an electric field can be supplied toalso a region which an electric field is difficult to reach in the caseof using only the conductor 404; thus, the subthreshold swing value(also referred to as an S value) of the transistor 492 can be small.Accordingly, the off-state current of the transistor 492 can be low.Alternatively, the transistor 492 does not necessarily include theconductor 413.

In the transistor illustrated in FIG. 1, the conductor 416 a and theconductor 416 b are not in contact with side surfaces of thesemiconductor 406 b. Thus, an electric field applied from the conductor404 functioning as a gate electrode to the side surfaces of thesemiconductor 406 b is less likely to be blocked by the conductor 416 aand the conductor 416 b. The conductor 416 a and the conductor 416 b arenot in contact with a top surface of the insulator 402. Thus, excessoxygen (oxygen) released from the insulator 402 is not consumed tooxidize the conductor 416 a and the conductor 416 b. Accordingly, in thetransistor illustrated in FIG. 1, excess oxygen (oxygen) released fromthe insulator 402 can be efficiently used to reduce oxygen vacancies inthe semiconductor 406 b. In other words, the transistor having thestructure illustrated in FIG. 1 has excellent electrical characteristicssuch as a high on-state current, high field-effect mobility, a smallsubthreshold swing value, and high reliability.

Although FIG. 1 and the like show an example where the conductor 416 aand the conductor 416 b which function as a source electrode and a drainelectrode are in contact with only a top surface of the semiconductor406 b, a transistor structure of one embodiment of the present inventionis not limited thereto. For example, the conductor 416 a and theconductor 416 b may be in contact with the top surface and the sidesurface of the semiconductor 406 b, the top surface of the insulator402, and the like.

The insulator 402 is preferably an insulator containing excess oxygen.

The insulator containing excess oxygen means an insulator from whichoxygen is released by heat treatment, for example. The silicon oxidecontaining excess oxygen means silicon oxide which can release oxygen byheat treatment or the like, for example. Therefore, the insulator 402 isan insulator in which oxygen can be moved. In other words, the insulator402 may be an insulator having an oxygen-transmitting property. Forexample, the insulator 402 may be an insulator having a higheroxygen-transmitting property than the semiconductor 406 a.

The insulator containing excess oxygen has a function of reducing oxygenvacancies in the semiconductor 406 b in some cases. Such oxygenvacancies form DOS in the semiconductor 406 b and serve as hole traps orthe like. In addition, hydrogen comes into the site of such oxygenvacancies and forms electrons serving as carriers. Therefore, byreducing the oxygen vacancies in the semiconductor 406 b, the transistor492 can have stable electrical characteristics.

Here, an insulator from which oxygen is released by heat treatment mayrelease oxygen, the amount of which is higher than or equal to 1×10¹⁸atoms/cm³, higher than or equal to 1×10¹⁹ atoms/cm³, or higher than orequal to 1×10²⁰ atoms/cm³ (converted into the number of oxygen atoms) inthermal desorption spectroscopy (TDS) analysis in the range of a surfacetemperature of 100° C. to 700° C. or 100° C. to 500° C.

Here, the method of measuring the amount of released oxygen using TDSanalysis is described below.

The total amount of released gas from a measurement sample in TDSanalysis is proportional to the integral value of the ion intensity ofthe released gas. Then, comparison with a reference sample is made,whereby the total amount of released gas can be calculated.

For example, the number of released oxygen molecules (N_(O2)) from ameasurement sample can be calculated according to the following formulausing the TDS results of a silicon substrate containing hydrogen at apredetermined density, which is a reference sample, and the TDS resultsof the measurement sample. Here, all gases having a mass-to-charge ratioof 32 which are obtained in the TDS analysis are assumed to originatefrom an oxygen molecule. Note that CH₃OH, which is a gas having themass-to-charge ratio of 32, is not taken into consideration because itis unlikely to be present. Furthermore, an oxygen molecule including anoxygen atom having a mass-to-charge ratio of 17 or 18 which is anisotope of an oxygen atom is also not taken into consideration becausethe proportion of such a molecule in the natural world is minimal.

Here, a formula N_(O2)=N_(H2)/S_(H2)×S_(O2)×α is used.

The value N_(H2) is obtained by conversion of the number of hydrogenmolecules desorbed from the reference sample into densities. The valueS_(H2) is the integral value of ion intensity in the case where thereference sample is subjected to the TDS analysis. Here, the referencevalue of the reference sample is set to N_(H2)/S_(H2). The value S_(O2)is the integral value of ion intensity when the measurement sample isanalyzed by TDS. The value a is a coefficient affecting the ionintensity in the TDS analysis. Refer to Japanese Published PatentApplication No. H6-275697 for details of the above formula. The amountof released oxygen was measured with a thermal desorption spectroscopyapparatus produced by ESCO Ltd., EMD-WA1000S/W using a silicon substratecontaining a certain amount of hydrogen atoms as the reference sample.

Furthermore, in the TDS analysis, oxygen is partly detected as an oxygenatom. The ratio between oxygen molecules and oxygen atoms can becalculated from the ionization rate of the oxygen molecules. Note that,since the above a includes the ionization rate of the oxygen molecules,the amount of the released oxygen atoms can also be estimated throughthe evaluation of the amount of the released oxygen molecules.

Note that N_(O2) is the amount of the released oxygen molecules. Theamount of released oxygen in the case of being converted into oxygenatoms is twice the amount of the released oxygen molecules.

Furthermore, the insulator from which oxygen is released by heattreatment may contain a peroxide radical. Specifically, the spin densityattributed to the peroxide radical is greater than or equal to 5×10¹⁷spins/cm³. Note that the insulator containing a peroxide radical mayhave an asymmetric signal with a g factor of approximately 2.01 in ESR.

The insulator containing excess oxygen may be formed using oxygen-excesssilicon oxide (SiO_(x) (X>2)). In the oxygen-excess silicon oxide(SiO_(x) (X>2)), the number of oxygen atoms per unit volume is more thantwice the number of silicon atoms per unit volume. The number of siliconatoms and the number of oxygen atoms per unit volume are measured byRutherford backscattering spectrometry (RBS).

The semiconductor 406 b can be electrically surrounded by an electricfield of the conductor 404 (a structure in which a semiconductor iselectrically surrounded by an electric field of a conductor is referredto as a surrounded channel (s-channel) structure). Therefore, a channelis formed in the entire semiconductor 406 b (bulk) in some cases. In thes-channel structure, a large amount of current can flow between a sourceand a drain of a transistor, so that a high on-state current can beobtained.

The s-channel structure is suitable for a miniaturized transistorbecause a high on-state current can be obtained. A semiconductor deviceincluding the miniaturized transistor can have a high integration degreeand high density. For example, the channel length of the transistor ispreferably less than or equal to 40 nm, more preferably less than orequal to 30 nm, still more preferably less than or equal to 20 nm andthe channel width of the transistor is preferably less than or equal to40 nm, more preferably less than or equal to 30 nm, still morepreferably less than or equal to 20 nm.

Note that the channel length refers to, for example, a distance betweena source (a source region or a source electrode) and a drain (a drainregion or a drain electrode) in a region where a semiconductor (or aportion where a current flows in a semiconductor when a transistor ison) and a gate electrode overlap with each other or a region where achannel is formed in a top view of the transistor. In one transistor,channel lengths in all regions are not necessarily the same. In otherwords, the channel length of one transistor is not limited to one valuein some cases. Therefore, in this specification, the channel length isany one of values, the maximum value, the minimum value, or the averagevalue in a region where a channel is formed.

A channel width refers to, for example, the length of a portion where asource and a drain face each other in a region where a semiconductor (ora portion where a current flows in a semiconductor when a transistor ison) and a gate electrode overlap with each other, or a region where achannel is formed in a top view of the transistor. In one transistor,channel widths in all regions do not necessarily have the same value. Inother words, a channel width of one transistor is not fixed to one valuein some cases. Therefore, in this specification, a channel width is anyone of values, the maximum value, the minimum value, or the averagevalue in a region where a channel is formed.

Note that depending on transistor structures, a channel width in aregion where a channel is formed actually (hereinafter referred to as aneffective channel width) is different from a channel width shown in atop view of a transistor (hereinafter referred to as an apparent channelwidth) in some cases. For example, in a transistor having athree-dimensional structure, an effective channel width is greater thanan apparent channel width shown in a top view of the transistor, and itsinfluence cannot be ignored in some cases. For example, in aminiaturized transistor having a three-dimensional structure, theproportion of a channel region formed in a side surface of asemiconductor is higher than the proportion of a channel region formedin a top surface of the semiconductor in some cases. In that case, aneffective channel width obtained when a channel is actually formed isgreater than an apparent channel width shown in the top view.

In a transistor having a three-dimensional structure, an effectivechannel width is difficult to measure in some cases. For example, toestimate an effective channel width from a design value, it is necessaryto assume that the shape of a semiconductor is known. Therefore, in thecase where the shape of a semiconductor is not known accurately, it isdifficult to measure an effective channel width accurately.

Therefore, in this specification, in a top view of a transistor, anapparent channel width that is a length of a portion where a source anda drain face each other in a region where a semiconductor and a gateelectrode overlap with each other is referred to as a surrounded channelwidth (SCW) in some cases. Furthermore, in this specification, in thecase where the term “channel width” is simply used, it may denote asurrounded channel width and an apparent channel width. Alternatively,in this specification, in the case where the term “channel width” issimply used, it may denote an effective channel width in some cases.Note that the values of a channel length, a channel width, an effectivechannel width, an apparent channel width, a surrounded channel width,and the like can be determined by obtaining and analyzing across-sectional TEM image and the like.

Note that in the case where field-effect mobility, a current value perchannel width, and the like of a transistor are obtained by calculation,a surrounded channel width may be used for the calculation. In thatcase, a value different from one in the case where an effective channelwidth is used for the calculation is obtained in some cases.

A structure of an oxide semiconductor which can be used as thesemiconductor 406 a, the semiconductor 406 b, the semiconductor 406 c,or the like is described below. In this specification, the trigonal andrhombohedral crystal systems are included in the hexagonal crystalsystem.

<Oxide Semiconductor Structure>

The structure of an oxide semiconductor is described below.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a c-axis alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor.Examples of a crystalline oxide semiconductor include a single crystaloxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor,and an nc-OS.

It is known that an amorphous structure is generally defined as beingmetastable and unfixed, and being isotropic and having no non-uniformstructure. In other words, an amorphous structure has a flexible bondangle and a short-range order but does not have a long-range order.

This means that an inherently stable oxide semiconductor cannot beregarded as a completely amorphous oxide semiconductor. Moreover, anoxide semiconductor that is not isotropic (e.g., an oxide semiconductorthat has a periodic structure in a microscopic region) cannot beregarded as a completely amorphous oxide semiconductor. Note that ana-like OS has a periodic structure in a microscopic region, but at thesame time has a void and has an unstable structure. For this reason, ana-like OS has physical properties similar to those of an amorphous oxidesemiconductor.

<CAAC-OS>

First, a CAAC-OS is described.

A CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, in the high-resolutionTEM image, a boundary between pellets, that is, a grain boundary is notclearly observed. Thus, in the CAAC-OS, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

A CAAC-OS observed with TEM is described below. FIG. 5A shows ahigh-resolution TEM image of a cross section of the CAAC-OS which isobserved from a direction substantially parallel to the sample surface.The high-resolution TEM image is obtained with a spherical aberrationcorrector function. The high-resolution TEM image obtained with aspherical aberration corrector function is particularly referred to as aCs-corrected high-resolution TEM image. The Cs-corrected high-resolutionTEM image can be obtained with, for example, an atomic resolutionanalytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 5B is an enlarged Cs-corrected high-resolution TEM image of aregion (1) in FIG. 5A. FIG. 5B shows that metal atoms are arranged in alayered manner in a pellet. Each metal atom layer has a configurationreflecting unevenness of a surface over which the CAAC-OS is formed(hereinafter, the surface is referred to as a formation surface) or atop surface of the CAAC-OS, and is arranged parallel to the formationsurface or the top surface of the CAAC-OS.

As shown in FIG. 5B, the CAAC-OS has a characteristic atomicarrangement. The characteristic atomic arrangement is denoted by anauxiliary line in FIG. 5C. FIGS. 5B and 5C prove that the size of apellet is greater than or equal to 1 nm, or greater than or equal to 3nm, and the size of a space caused by tilt of the pellets isapproximately 0.8 nm. Therefore, the pellet can also be referred to as ananocrystal (nc). A CAAC-OS can be referred to as an oxide semiconductorincluding c-axis aligned nanocrystals (CANC).

Here, according to the Cs-corrected high-resolution TEM images, theschematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120is illustrated by such a structure in which bricks or blocks are stacked(see FIG. 5D). The part in which the pellets are tilted as observed inFIG. 5C corresponds to a region 5161 shown in FIG. 5D.

FIG. 6A shows a Cs-corrected high-resolution TEM image of a plane of theCAAC-OS observed from a direction substantially perpendicular to thesample surface. FIGS. 6B, 6C, and 6D are enlarged Cs-correctedhigh-resolution TEM images of regions (1), (2), and (3) in FIG. 6A,respectively. FIGS. 6B, 6C, and 6D indicate that metal atoms arearranged in a triangular, quadrangular, or hexagonal configuration in apellet. However, there is no regularity of arrangement of metal atomsbetween different pellets.

Next, a CAAC-OS analyzed by X-ray diffraction (XRD) is described. Forexample, when the structure of a CAAC-OS including an InGaZnO₄ crystalis analyzed by an out-of-plane method, a peak appears at a diffractionangle (2θ) of around 31° as shown in FIG. 7A. This peak is derived fromthe (009) plane of the InGaZnO₄ crystal, which indicates that crystalsin the CAAC-OS have c-axis alignment, and that the c-axes are aligned ina direction substantially perpendicular to the formation surface or thetop surface of the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-planemethod, another peak may appear when 2θ is around 36°, in addition tothe peak at 2θ of around 31°. The peak at 2θ of around 36° indicatesthat a crystal having no c-axis alignment is included in part of theCAAC-OS. It is preferable that in the CAAC-OS analyzed by anout-of-plane method, a peak appear when 2θ is around 31° and that a peaknot appear when 2θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray is incident on a sample in a directionsubstantially perpendicular to the c-axis, a peak appears when 2θ isaround 56°. This peak is attributed to the (110) plane of the InGaZnO₄crystal. In the case of the CAAC-OS, when analysis (φ scan) is performedwith 2θ fixed at around 56° and with the sample rotated using a normalvector of the sample surface as an axis (φ axis), as shown in FIG. 7B, apeak is not clearly observed. In contrast, in the case of a singlecrystal oxide semiconductor of InGaZnO₄, when 0 scan is performed with2θ fixed at around 56°, as shown in FIG. 7C, six peaks which are derivedfrom crystal planes equivalent to the (110) plane are observed.Accordingly, the structural analysis using XRD shows that the directionsof a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the sample surface, a diffraction pattern (also referred toas a selected-area transmission electron diffraction pattern) shown inFIG. 8A can be obtained. In this diffraction pattern, spots derived fromthe (009) plane of an InGaZnO₄ crystal are included. Thus, the electrondiffraction also indicates that pellets included in the CAAC-OS havec-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 8B shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 8B, a ring-like diffraction pattern isobserved. Thus, the electron diffraction also indicates that the a-axesand b-axes of the pellets included in the CAAC-OS do not have regularalignment. The first ring in FIG. 8B is considered to be derived fromthe (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal.The second ring in FIG. 8B is considered to be derived from the (110)plane and the like.

As described above, the CAAC-OS is an oxide semiconductor with highcrystallinity. Entry of impurities, generation of defects, or the likemight decrease the crystallinity of an oxide semiconductor. This meansthat the CAAC-OS has small amounts of impurities and defects (e.g.,oxygen vacancies).

Note that the impurity means an element other than the main componentsof the oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically, siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities ordefects might be changed by light, heat, or the like. The impuritycontained in the oxide semiconductor might serve as a carrier trap orserve as a carrier generation source. Furthermore, oxygen vacancies inthe oxide semiconductor serve as carrier traps or serve as carriergeneration sources when hydrogen is captured therein.

The CAAC-OS having small amounts of impurities and oxygen vacancies isan oxide semiconductor with low carrier density (specifically, lowerthan 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³, further preferablylower than 1×10¹⁰/cm³, and is higher than or equal to 1×10⁻⁹/cm³). Suchan oxide semiconductor is referred to as a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. A CAAC-OShas a low impurity concentration and a low density of defect states.Thus, the CAAC-OS can be referred to as an oxide semiconductor havingstable characteristics.

<nc-OS>

Next, an nc-OS is described.

An nc-OS has a region in which a crystal part is observed and a regionin which a crystal part is not clearly observed in a high-resolution TEMimage. In most cases, the size of a crystal part included in the nc-OSis greater than or equal to 1 nm and less than or equal to 10 nm, orgreater than or equal to 1 nm and less than or equal to 3 nm. Note thatan oxide semiconductor including a crystal part with a size greater thanor equal to 10 nm and less than or equal to 100 nm is referred to as amicrocrystalline oxide semiconductor in some cases. In a high-resolutionTEM image of the nc-OS, for example, a grain boundary is not clearlyobserved in some cases. Note that there is a possibility that the originof the nanocrystal is the same as that of a pellet in a CAAC-OS.Therefore, a crystal part of the nc-OS may be referred to as a pellet inthe following description.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has periodic atomic arrangement. There is noregularity of crystal orientation between different pellets in thenc-OS. Thus, the orientation of the whole film is not observed.Accordingly, the nc-OS cannot be distinguished from an a-like OS or anamorphous oxide semiconductor, depending on an analysis method. Forexample, when the nc-OS is analyzed by an out-of-plane method using anX-ray having a diameter larger than the size of a pellet, a peak whichshows a crystal plane does not appear. Furthermore, a diffractionpattern like a halo pattern is observed when the nc-OS is subjected toelectron diffraction using an electron beam with a probe diameter (e.g.,50 nm or larger) that is larger than the size of a pellet. Meanwhile,spots appear in a nanobeam electron diffraction pattern of the nc-OSwhen an electron beam with a probe diameter close to or smaller than thesize of a pellet is used. Moreover, in a nanobeam electron diffractionpattern of the nc-OS, regions with high luminance in a circular (ring)pattern are shown in some cases. Also in a nanobeam electron diffractionpattern of the nc-OS, a plurality of spots is shown in a ring-likeregion in some cases.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including random aligned nanocrystals (RANC) oran oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as comparedwith an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an amorphous oxidesemiconductor. Note that there is no regularity of crystal orientationbetween different pellets in the nc-OS. Therefore, the nc-OS has ahigher density of defect states than the a-like OS and the CAAC-OS.

<a-like OS>

An a-like OS is an oxide semiconductor having a structure intermediatebetween the nc-OS and the amorphous oxide semiconductor.

In a high-resolution TEM image of the a-like OS, a void may be observed.Furthermore, in the high-resolution TEM image, there are a region wherea crystal part is clearly observed and a region where a crystal part isnot observed.

The a-like OS has an unstable structure because it contains a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS (sample A), an nc-OS (sample B), and a CAAC-OS (sample C)are prepared as samples subjected to electron irradiation. Each of thesamples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

Note that which part is regarded as a crystal part is determined asfollows. It is known that a unit cell of an InGaZnO₄ crystal has astructure in which nine layers including three In—O layers and sixGa—Zn—O layers are stacked in the c-axis direction. The distance betweenthe adjacent layers is equivalent to the lattice spacing on the (009)plane (also referred to as d value). The value is calculated to be 0.29nm from crystal structural analysis. Accordingly, a portion where thelattice spacing between lattice fringes is greater than or equal to 0.28nm and less than or equal to 0.30 nm is regarded as a crystal part ofInGaZnO₄. Each of lattice fringes corresponds to the a-b plane of theInGaZnO₄ crystal.

FIG. 9 shows change in the average size of crystal parts (at 22 pointsto 45 points) in each sample. Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 9 indicates that thecrystal part size in the a-like OS increases with an increase in thecumulative electron dose. Specifically, as shown by (1) in FIG. 9, acrystal part of approximately 1.2 nm (also referred to as an initialnucleus) at the start of TEM observation grows to a size ofapproximately 2.6 nm at a cumulative electron dose of 4.2×10⁸ e⁻/nm². Incontrast, the crystal part size in the nc-OS and the CAAC-OS showslittle change from the start of electron irradiation to a cumulativeelectron dose of 4.2×10⁸ e⁻/hm². Specifically, as shown by (2) and (3)in FIG. 9, the average crystal sizes in an nc-OS and a CAAC-OS areapproximately 1.4 nm and approximately 2.1 nm, respectively, regardlessof the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS is inducedby electron irradiation. In contrast, in the nc-OS and the CAAC-OS,growth of the crystal part is hardly induced by electron irradiation.Therefore, the a-like OS has an unstable structure as compared with thenc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit contains a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of the single crystal oxide semiconductorhaving the same composition. Note that it is difficult to deposit anoxide semiconductor having a density of lower than 78% of the density ofthe single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomicratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor having acertain composition cannot exist in a single crystal structure. In thatcase, single crystal oxide semiconductors with different compositionsare combined at an adequate ratio, which makes it possible to calculatedensity equivalent to that of a single crystal oxide semiconductor withthe desired composition. The density of a single crystal oxidesemiconductor having the desired composition can be calculated using aweighted average according to the combination ratio of the singlecrystal oxide semiconductors with different compositions. Note that itis preferable to use as few kinds of single crystal oxide semiconductorsas possible to calculate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedlayer including two or more films of an amorphous oxide semiconductor,an a-like OS, an nc-OS, and a CAAC-OS, for example.

The above oxide semiconductor can be used as the semiconductor 406 a,the semiconductor 406 b, the semiconductor 406 c, or the like.

Next, the other components of a semiconductor which can be used as thesemiconductor 406 a, the semiconductor 406 b, the semiconductor 406 c,or the like are described.

The semiconductor 406 b is an oxide semiconductor containing indium, forexample. An oxide semiconductor can have high carrier mobility (electronmobility) by containing indium, for example. The semiconductor 406 bpreferably contains an element M. The element M is preferably aluminum,gallium, yttrium, tin, or the like. Other elements which can be used asthe element M are boron, silicon, titanium, iron, nickel, germanium,yttrium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium,tantalum, tungsten, and the like. Note that two or more of the aboveelements may be used in combination as the element M. The element M isan element having high bonding energy with oxygen, for example. Theelement M is an element whose bonding energy with oxygen is higher thanthat of indium. The element M is an element that can increase the energygap of the oxide semiconductor, for example. Furthermore, thesemiconductor 406 b preferably contains zinc. When the oxidesemiconductor contains zinc, the oxide semiconductor is easily to becrystallized, for example.

Note that the semiconductor 406 b is not limited to the oxidesemiconductor containing indium. The semiconductor 406 b may be, forexample, an oxide semiconductor which does not contain indium andcontains zinc, an oxide semiconductor which does not contain indium andcontains gallium, or an oxide semiconductor which does not containindium and contains tin, e.g., a zinc tin oxide or a gallium tin oxide.

For the semiconductor 406 b, an oxide with a wide energy gap may beused. For example, the energy gap of the semiconductor 406 b is greaterthan or equal to 2.5 eV and less than or equal to 4.2 eV, preferablygreater than or equal to 2.8 eV and less than or equal to 3.8 eV, morepreferably greater than or equal to 3 eV and less than or equal to 3.5eV.

For example, the semiconductor 406 a and the semiconductor 406 c includeone or more elements other than oxygen included in the semiconductor 406b. Since the semiconductor 406 a and the semiconductor 406 c eachinclude one or more elements other than oxygen included in thesemiconductor 406 b, an interface state is less likely to be formed atthe interface between the semiconductor 406 a and the semiconductor 406b and the interface between the semiconductor 406 b and thesemiconductor 406 c.

The semiconductor 406 a, the semiconductor 406 b, and the semiconductor406 c preferably include at least indium. In the case of using anIn-M-Zn oxide as the semiconductor 406 a, when the summation of In and Mis assumed to be 100 atomic %, the proportions of In and M arepreferably set to be less than 50 atomic % and greater than 50 atomic %,respectively, more preferably less than 25 atomic % and greater than 75atomic %, respectively. In the case of using an In-M-Zn oxide as thesemiconductor 406 b, when the summation of In and M is assumed to be 100atomic %, the proportions of In and M are preferably set to be greaterthan 25 atomic % and less than 75 atomic %, respectively, morepreferably greater than 34 atomic % and less than 66 atomic %,respectively. In the case of using an In-M-Zn oxide as the semiconductor406 c, when the summation of In and M is assumed to be 100 atomic %, theproportions of In and M are preferably set to be less than 50 atomic %and greater than 50 atomic %, respectively, more preferably less than 25atomic % and greater than 75 atomic %, respectively. Note that thesemiconductor 406 c may be an oxide that is a type the same as that ofthe semiconductor 406 a.

As the semiconductor 406 b, an oxide having an electron affinity higherthan those of the semiconductors 406 a and 406 c is used. For example,as the semiconductor 406 b, an oxide having an electron affinity higherthan those of the semiconductors 406 a and 406 c by 0.07 eV or higherand 1.3 eV or lower, preferably 0.1 eV or higher and 0.7 eV or lower,more preferably 0.15 eV or higher and 0.4 eV or lower is used. Note thatthe electron affinity refers to an energy difference between the vacuumlevel and the bottom of the conduction band.

An indium gallium oxide has a small electron affinity and a highoxygen-blocking property. Therefore, the semiconductor 406 c preferablyincludes an indium gallium oxide. The gallium atomic ratio [Ga/(In+Ga)]is, for example, higher than or equal to 70%, preferably higher than orequal to 80%, more preferably higher than or equal to 90%.

At this time, when a gate voltage is applied, a channel is formed in thesemiconductor 406 b having the highest electron affinity in thesemiconductor 406 a, the semiconductor 406 b, and the semiconductor 406c.

Here, in some cases, there is a mixed region of the semiconductor 406 aand the semiconductor 406 b between the semiconductor 406 a and thesemiconductor 406 b. Furthermore, in some cases, there is a mixed regionof the semiconductor 406 b and the semiconductor 406 c between thesemiconductor 406 b and the semiconductor 406 c. The mixed region has alow interface state density. For that reason, the stack of thesemiconductor 406 a, the semiconductor 406 b, and the semiconductor 406c has a band diagram where energy at each interface and in the vicinityof the interface is changed continuously (continuous junction). Notethat FIG. 10A is a cross-sectional view in which the semiconductor 406a, the semiconductor 406 b, and the semiconductor 406 c are stacked inthis order. FIG. 10B shows energy (Ec) of the bottom of the conductionband corresponding to dashed-dotted line P1-P2 in FIG. 10A when thesemiconductor 406 c has a higher electron affinity than thesemiconductor 406 a. FIG. 10C shows energy (Ec) of the bottom of theconduction band corresponding to dashed-dotted line P1-P2 in FIG. 10Awhen the semiconductor 406 c has a lower electron affinity than thesemiconductor 406 a.

At this time, electrons move mainly in the semiconductor 406 b, not inthe semiconductor 406 a and the semiconductor 406 c. As described above,when the interface state density at the interface between thesemiconductor 406 a and the semiconductor 406 b and the interface statedensity at the interface between the semiconductor 406 b and thesemiconductor 406 c are decreased, electron movement in thesemiconductor 406 b is less likely to be inhibited and the on-statecurrent of the transistor 492 can be increased.

As factors of inhibiting electron movement are decreased, the on-statecurrent of the transistor 492 can be increased. For example, in the casewhere there is no factor of inhibiting electron movement, electrons areassumed to be efficiently moved. Electron movement is inhibited, forexample, in the case where physical unevenness of a channel formationregion is large.

Therefore, to increase the on-state current of the transistor 492, forexample, root mean square (RMS) roughness with a measurement area of 1μm×1 μm of a top surface or a bottom surface of the semiconductor 406 b(a formation surface; here, the semiconductor 406 a) is less than 1 nm,preferably less than 0.6 nm, more preferably less than 0.5 nm, stillmore preferably less than 0.4 nm. The average surface roughness (alsoreferred to as Ra) with the measurement area of 1 μm×1 μm is less than 1nm, preferably less than 0.6 nm, more preferably less than 0.5 nm, stillmore preferably less than 0.4 nm. The maximum difference (Peak−Valley(P−V)) with the measurement area of 1 μm×1 μm is less than 10 nm,preferably less than 9 nm, more preferably less than 8 nm, still morepreferably less than 7 nm. RMS roughness, Ra, and P−V can be measuredusing a scanning probe microscope SPA-500 manufactured by SII NanoTechnology Inc.

The electron movement is also inhibited, for example, in the case wherethe density of defect states is high in a region where a channel isformed.

For example, in the case were the semiconductor 406 b contains oxygenvacancies (also denoted by Vo), donor levels are formed by entry ofhydrogen into sites of oxygen vacancies in some cases. A state in whichhydrogen enters sites of oxygen vacancies are denoted by VoH in thefollowing description in some cases. VoH is a factor of decreasing theon-state current of the transistor 492 because VoH scatters electrons.Note that sites of oxygen vacancies become more stable by entry ofoxygen than by entry of hydrogen. Thus, by decreasing oxygen vacanciesin the semiconductor 406 b, the on-state current of the transistor 492can be increased in some cases.

To decrease oxygen vacancies in the semiconductor 406 b, for example,there is a method in which excess oxygen in the insulator 402 is movedto the semiconductor 406 b through the semiconductor 406 a. In thiscase, the semiconductor 406 a is preferably a layer having anoxygen-transmitting property (a layer through which oxygen passes or istransmitted).

Oxygen is released from the insulator 402 and taken into thesemiconductor 406 a by heat treatment or the like. In some cases, oxygenexists and is apart from atomics in the semiconductor 406 a, or existsand is bonded to oxygen or the like. As the density becomes lower, i.e.,the number of spaces between the atoms becomes larger, the semiconductor406 a has a higher oxygen-transmitting property. For example, in thecase where the semiconductor 406 a has a layered crystal structure andoxygen movement in which oxygen crosses the layer is less likely tooccur, the semiconductor 406 a is preferably a layer having lowcrystallinity as appropriate.

The semiconductor 406 a preferably has crystallinity such that excessoxygen (oxygen) is transmitted so that excess oxygen (oxygen) releasedfrom the insulator 402 reaches the semiconductor 406 b. For example, inthe case where the semiconductor 406 a is a CAAC-OS, a structure inwhich a space is partly provided in the layer is preferably employedbecause when the whole layer becomes CAAC, transmission of excess oxygen(oxygen) is difficult.

In the case where the transistor 492 has an s-channel structure, achannel is formed in the whole of the semiconductor 406 b, as shown inFIG. 1. Therefore, as the semiconductor 406 b has a larger thickness, achannel region becomes larger. In other words, the thicker thesemiconductor 406 b is, the larger the on-state current of thetransistor 492 is. For example, the semiconductor 406 b has a regionwith a thickness of greater than or equal to 20 nm, preferably greaterthan or equal to 40 nm, more preferably greater than or equal to 60 nm,still more preferably greater than or equal to 100 nm. Note that thesemiconductor 406 b has a region with a thickness of, for example, lessthan or equal to 300 nm, preferably less than or equal to 200 nm, morepreferably less than or equal to 150 nm because the productivity of thesemiconductor device might be decreased.

Moreover, the thickness of the semiconductor 406 c is preferably assmall as possible to increase the on-state current of the transistor492. The thickness of the semiconductor 406 c is less than 10 nm,preferably less than or equal to 5 nm, more preferably less than orequal to 3 nm, for example. Meanwhile, the semiconductor 406 c has afunction of blocking entry of elements other than oxygen (such ashydrogen and silicon) included in the adjacent insulator into thesemiconductor 406 b where a channel is formed. For this reason, it ispreferable that the semiconductor 406 c have a certain thickness. Thethickness of the semiconductor 406 c is greater than or equal to 0.3 nm,preferably greater than or equal to 1 nm, more preferably greater thanor equal to 2 nm, for example. The semiconductor 406 c preferably has anoxygen blocking property to suppress outward diffusion of oxygenreleased from the insulator 402 and the like.

To improve reliability, preferably, the thickness of the semiconductor406 a is large and the thickness of the semiconductor 406 c is small.For example, the semiconductor 406 a has a region with a thickness of,for example, greater than or equal to 10 nm, preferably greater than orequal to 20 nm, more preferably greater than or equal to 40 nm, stillmore preferably greater than or equal to 60 nm. When the thickness ofthe semiconductor 406 a is made large, a distance from an interfacebetween the adjacent insulator and the semiconductor 406 a to thesemiconductor 406 b in which a channel is formed can be large. Since theproductivity of the semiconductor device might be decreased, thesemiconductor 406 a has a region with a thickness of, for example, lessthan or equal to 200 nm, preferably less than or equal to 120 nm, morepreferably less than or equal to 80 nm.

The silicon concentration at an interface between the semiconductor 406b in which a channel is formed and the semiconductor 406 a adjacent tothe semiconductor 406 b significantly influences the electriccharacteristics of the transistor 492; therefore, it is preferable thatthe silicon concentration at the interface be sufficiently low.

For example, a region with a silicon concentration of lower than 1×10¹⁹atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, more preferably lowerthan 2×10¹⁸ atoms/cm³ which is measured by secondary ion massspectrometry (SIMS) is provided between the semiconductor 406 b and thesemiconductor 406 a. A region with a silicon concentration of lower than1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, morepreferably lower than 2×10¹⁸ atoms/cm³ which is measured by SIMS isprovided between the semiconductor 406 b and the semiconductor 406 c.

It is preferable to reduce the concentration of hydrogen in thesemiconductor 406 a and the semiconductor 406 c in order to reduce theconcentration of hydrogen in the semiconductor 406 b. The semiconductor406 a and the semiconductor 406 c each have a region in which theconcentration of hydrogen measured by SIMS is lower than or equal to2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³,more preferably lower than or equal to 1×10¹⁹ atoms/cm³, still morepreferably lower than or equal to 5×10¹⁸ atoms/cm³. It is preferable toreduce the concentration of nitrogen in the semiconductor 406 a and thesemiconductor 406 c in order to reduce the concentration of nitrogen inthe semiconductor 406 b. The semiconductor 406 a and the semiconductor406 c each have a region in which the concentration of nitrogen measuredby SIMS is lower than 5×10¹⁹ atoms/cm³, preferably lower than or equalto 5×10¹⁸ atoms/cm³, more preferably lower than or equal to 1×10¹⁸atoms/cm³, still more preferably lower than or equal to 5×10¹⁷atoms/cm³.

The above three-layer structure is an example. For example, a two-layerstructure without the semiconductor 406 a or the semiconductor 406 c maybe employed. A four-layer structure in which any one of thesemiconductors described as examples of the semiconductor 406 a, thesemiconductor 406 b, and the semiconductor 406 c is provided below orover the semiconductor 406 a or below or over the semiconductor 406 cmay be employed. An n-layer structure (n is an integer of 5 or more) inwhich any one of the semiconductors described as examples of thesemiconductor 406 a, the semiconductor 406 b, and the semiconductor 406c is provided at two or more of the following positions: over thesemiconductor 406 a, below the semiconductor 406 a, over thesemiconductor 406 c, and below the semiconductor 406 c.

At least part (or all) of the conductor 416 a (and/or the conductor 416b) is provided on at least part (or all) of a surface, a side surface, atop surface, and/or a bottom surface of a semiconductor, e.g., thesemiconductor 406 b.

Alternatively, at least part (or all) of the conductor 416 a (and/or theconductor 416 b) is in contact with at least part (or all) of a surface,a side surface, a top surface, and/or a bottom surface of asemiconductor, e.g., the semiconductor 406 b. Alternatively, at leastpart (or all) of the conductor 416 a (and/or the conductor 416 b) is incontact with at least part (or all) of a semiconductor, e.g., thesemiconductor 406 b.

Alternatively, at least part (or all) of the conductor 416 a (and/or theconductor 416 b) is electrically connected to at least part (or all) ofa surface, a side surface, a top surface, and/or a bottom surface of asemiconductor, e.g., the semiconductor 406 b. Alternatively, at leastpart (or all) of the conductor 416 a (and/or the conductor 416 b) iselectrically connected to at least part (or all) of a semiconductor,e.g., the semiconductor 406 b.

Alternatively, at least part (or all) of the conductor 416 a (and/or theconductor 416 b) is provided near at least part (or all) of a surface, aside surface, a top surface, and/or a bottom surface of a semiconductor,e.g., the semiconductor 406 b. Alternatively, at least part (or all) ofthe conductor 416 a (and/or the conductor 416 b) is provided near atleast part (or all) of a semiconductor, e.g., the semiconductor 406 b.

Alternatively, at least part (or all) of the conductor 416 a (and/or theconductor 416 b) is provided to be adjacent to at least part (or all) ofa surface, a side surface, a top surface, and/or a bottom surface of asemiconductor, e.g., the semiconductor 406 b. Alternatively, at leastpart (or all) of the conductor 416 a (and/or the conductor 416 b) isprovided to be adjacent to at least part (or all) of a semiconductor,e.g., the semiconductor 406 b.

Alternatively, at least part (or all) of the conductor 416 a (and/or theconductor 416 b) is provided obliquely above at least part (or all) of asurface, a side surface, a top surface, and/or a bottom surface of asemiconductor, e.g., the semiconductor 406 b. Alternatively, at leastpart (or all) of the conductor 416 a (and/or the conductor 416 b) isprovided obliquely above at least part (or all) of a semiconductor,e.g., the semiconductor 406 b.

Alternatively, at least part (or all) of the conductor 416 a (and/or theconductor 416 b) is provided above at least part (or all) of a surface,a side surface, a top surface, and/or a bottom surface of asemiconductor, e.g., the semiconductor 406 b. Alternatively, at leastpart (or all) of the conductor 416 a (and/or the conductor 416 b) isprovided above at least part (or all) of a semiconductor, e.g., thesemiconductor 406 b.

Although FIG. 1 and the like show an example where the conductor 416 aand the conductor 416 b which function as a source electrode and a drainelectrode are in contact with only a top surface of the semiconductor406 b, a transistor structure of one embodiment of the present inventionis not limited thereto. For example, the conductor 416 a and theconductor 416 b may be in contact with the top surface and a sidesurface of the semiconductor 406 b, a top surface of the insulator 402,and the like.

In the transistor illustrated in FIG. 1, the conductor 416 a and theconductor 416 b are not in contact with side surfaces of thesemiconductor 406 b. Thus, an electric field applied from the conductor404 functioning as a gate electrode to the side surfaces of thesemiconductor 406 b is less likely to be blocked by the conductor 416 aand the conductor 416 b. The conductor 416 a and the conductor 416 b arenot in contact with a top surface of the insulator 402. Thus, excessoxygen (oxygen) released from the insulator 402 is not consumed tooxidize the conductor 416 a and the conductor 416 b. Accordingly, in thetransistor illustrated in FIG. 1, excess oxygen (oxygen) released fromthe insulator 402 can be efficiently used to reduce oxygen vacancies inthe semiconductor 406 b. In other words, the transistor with thestructure illustrated in FIG. 1 has excellent electrical characteristicssuch as a high on-state current, high field-effect mobility, a smallsubthreshold swing value, and high reliability.

The insulator 401 illustrated in FIG. 1 and the like is provided betweenthe transistors 491 and 492 and the like. As the insulator 401, an oxidecontaining aluminum, e.g., aluminum oxide, is used. The insulator 401blocks oxygen and hydrogen, and aluminum oxide whose density is lowerthan 3.2 g/cm³ is preferable because it has a particularly high functionof blocking hydrogen. Alternatively, aluminum oxide with lowcrystallinity is preferable because its function of blocking hydrogen isparticularly high.

For example, in the case where the transistor 491 is a silicontransistor, electrical characteristics of the transistor may be improvedbecause dangling bonds of silicon can be reduced by supplying hydrogenfrom the outside. The supply of hydrogen may be performed by heattreatment under an atmosphere containing hydrogen, for example.Alternatively, for example, an insulator containing hydrogen is providedin the vicinity of the transistor 491 and heat treatment is performed,so that the hydrogen may be diffused and supplied to the transistor 491.Specifically, an insulator 478 over the transistor 491 is preferably aninsulator containing hydrogen. Note that the insulator 478 may have asingle-layer structure or a stacked-layer structure. For example, astacked-layer structure including silicon oxynitride or silicon oxide,and silicon nitride oxide or silicon nitride may be used.

An insulator containing hydrogen may release hydrogen, the amount ofwhich is larger than or equal to 1×10¹⁸ atoms/cm³, larger than or equalto 1×10¹⁹ atoms/cm³, or larger than or equal to 1×10²⁰ atoms/cm³ in TDSanalysis (converted into the number of hydrogen atoms) in the range of asurface temperature of 100° C. to 700° C. or 100° C. to 500° C.

Hydrogen diffused from the insulator 478 might reach the vicinity of thetransistor 492 passing through the conductor 469 provided in an openingof the insulator 478, the conductor 474 a and the conductor 474 b overthe insulator 478, a conductor 421 over the conductor 474, and the like.However, the amount of hydrogen that reaches the transistor 492 is smallbecause the insulator 401 has a function of blocking hydrogen. Hydrogenserves as a carrier trap or a carrier generation source in an oxidesemiconductor and causes deterioration of electrical characteristics ofthe transistor 492 in some cases. Therefore, blocking hydrogen by theinsulator 401 is important to improve performance and reliability of thesemiconductor device. Note that a conductor embedded in an opening,e.g., the conductor 469, has a function of electrically connectingelements such as transistors and capacitors.

On the other hand, for example, by supplying oxygen to the transistor492 from the outside, oxygen vacancies in the oxide semiconductor can bereduced; thus, electrical characteristics of the transistor are improvedin some cases. The supply of oxygen may be performed by heat treatmentunder an atmosphere containing oxygen, for example. Alternatively, forexample, an insulator containing excess oxygen (oxygen) is provided inthe vicinity of the transistor 492 and heat treatment is performed, sothat the oxygen may be diffused and supplied to the transistor 492.Here, as the insulator 402 of the transistor 492, an insulatorcontaining excess oxygen is used.

Diffused oxygen might reach the transistor 491 through layers; however,since the insulator 401 has a function of blocking oxygen, the amount ofoxygen which reaches the transistor 491 is small. In the case where thetransistor 491 is a silicon transistor, entry of oxygen into siliconmight be a factor of decreasing crystallinity of silicon or inhibitingcarrier movement. Therefore, blocking oxygen by the insulator 401 isimportant to improve performance and reliability of the semiconductordevice.

In FIG. 1 and the like, the semiconductor device preferably includes aninsulator 408 over the transistor 492. The insulator 408 has a functionof blocking oxygen and hydrogen. For the insulator 408, the descriptionof the insulator 401 is referred to, for example. The insulator 408 has,for example, a higher function of blocking oxygen and hydrogen than thesemiconductor 406 a and/or the semiconductor 406 c.

When the semiconductor device includes the insulator 408, outwarddiffusion of oxygen from the transistor 492 can be suppressed.Consequently, excess oxygen (oxygen) contained in the insulator 402 andthe like can be effectively supplied to the transistor 492. Since theinsulator 408 blocks entry of impurities including hydrogen from layersabove the insulator 408 or the outside of the semiconductor device,deterioration of the electrical characteristics of the transistor 492due to the entry of impurities can be suppressed.

Although in the above description, the insulator 401 and/or theinsulator 408 is described separately from the transistor 492 forconvenience, the insulator 401 and/or the insulator 408 may be part ofthe transistor 492.

A conductor 421 a and a conductor 417 function as electrodes of thecapacitor 493. An insulator 419 is provided between the conductor 421 aand the conductor 417.

The semiconductor device may include an insulator 418 over the insulator408. Furthermore, the semiconductor device may include a conductor 424 aand a conductor 424 b which are electrically connected to the transistor492 through a conductor 421 a and a conductor 421 b, respectively,provided in openings of the insulator 418.

<Method of Manufacturing Semiconductor Device>

Next, a method of manufacturing the semiconductor device illustrated inFIG. 1 is described with reference to FIGS. 11A to 11C, FIGS. 12A and12B, FIG. 13, FIG. 14, FIG. 15, FIG. 16, and FIG. 17. The left view inFIG. 1 illustrates the cross sections of the transistor 491 and thetransistor 492 in the channel length direction. The right view in FIG. 1illustrates a cross section taken along the dashed-dotted line (a-a′) inthe left view in FIG. 1. That is, the right view is a cross-sectionalview, which is across the center of the conductor 404, in the channelwidth direction of the transistor 492.

First, the transistor 491 is formed over the semiconductor substrate400. The insulator 478 is formed over the transistor 491, and subjectedto chemical mechanical polishing (CMP) treatment to planarize itsinsulating surface. Although the CMP treatment is used here, othertreatment may be employed as the planarization treatment. Alternatively,the CMP treatment may be combined with etching (dry etching or wetetching) treatment or plasma treatment (see FIG. 11A).

As the semiconductor substrate 400, a single crystal semiconductorsubstrate or a polycrystalline semiconductor substrate made of siliconor silicon carbide; a compound semiconductor substrate made of silicongermanium and the like; a silicon-on-insulator (SOI) substrate; or thelike can be used.

A contact hole is formed in the insulator 478 by photolithography anddry etching. Next, the contact hole is filled with the conductor 469 andCMP treatment is performed to remove the conductor over the insulator478. The conductor is formed by, for example, a sputtering method, achemical vapor deposition (CVD) method, or an atomic layer deposition(ALD) method. Although CMP treatment is used here to remove theconductor over the insulator 478, other treatment may be employed.Alternatively, the CMP treatment may be combined with etching (dryetching or wet etching) treatment or plasma treatment. Then, theconductor 474 is deposited on the insulator 478, and the conductor 474 aand 474 b are formed by photolithography and dry etching. The conductor474 is formed by a sputtering method, a CVD method, an ALD method, orthe like. The insulator 480 is formed over the conductor 474, andsubjected to CMP treatment to planarize its insulating surface. AlthoughCMP treatment is used here as planarization treatment, other treatmentmay be employed. Alternatively, the CMP treatment may be combined withetching (dry etching or wet etching) treatment or plasma treatment (seeFIG. 11B).

Next, a conductor is deposited on the insulator 480, and the conductor417 is formed by photolithography and dry etching. The conductor 417 isformed by a sputtering method, a CVD method, an ALD method, or the like.Then, the insulator 482 is formed, and subjected to CMP treatment toplanarize its insulating surface. A top surface of the conductor 417 ispartly exposed by the planarization treatment or the like. Specifically,the planarization treatment is performed such that the top surface ofthe conductor 417 and a top surface of the insulator 482 lie in the sameplane. Exposing part of a top surface of a conductor in this way, forexample, removing an insulator and the like such that the top surface ofthe conductor is parallel to a reference surface (e.g., a rear surfaceof a substrate), is called “top surface exposure” in some cases.

Although the CMP treatment is used here, other treatment may be employedas the planarization treatment. Alternatively, the CMP treatment may becombined with etching (dry etching or wet etching) treatment or plasmatreatment (see FIG. 11C).

Next, a conductor is deposited on the insulator 482, and the conductor413 and the conductor 445 are formed by photolithography and dryetching. The conductor may be deposited by sputtering, CVD, ALD, and thelike. Then, the insulator 484 is formed and subjected to CMP treatmentto planarize its insulating surface. At this time, the top surfaces ofthe conductor 413 and the conductor 445 are exposed such that the topsurfaces and a top surface of the insulator 484 lie in the same plane.The conductor 417 and the conductor 445 are electrically connected toeach other. Although the CMP treatment is used here, other treatment maybe employed as the planarization treatment. Alternatively, the CMPtreatment may be combined with etching (dry etching or wet etching)treatment or plasma treatment (see FIG. 12A).

Next, the insulator 401 and the insulator 402 are formed. The insulator401 and the insulator 402 may be formed by a sputtering method, a CVDmethod, an MBE method, a PLD method, an ALD method, or the like. Notethat here, an example where the top surface of the insulator 402 isplanarized by CMP treatment or the like is described. By planarizing thetop surface of the insulator 402, the subsequent steps can be performedeasily, and the yield of the transistor 492 can be increased. Forexample, by a CMP method, the RMS roughness of the insulator 402 is lessthan or equal to 1 nm, preferably less than or equal to 0.5 nm, furtherpreferably less than or equal to 0.3 nm. Ra with the measurement area of1 μm×1 μm is less than 1 nm, preferably less than 0.6 nm, morepreferably less than 0.5 nm, still more preferably less than 0.4 nm. P−Vwith the measurement area of 1 μm×1 μm is less than 10 nm, preferablyless than 9 nm, more preferably less than 8 nm, still more preferablyless than 7 nm. The transistor 492 of one embodiment of the presentinvention is not limited to a transistor when the top surface of theinsulator 402 is planarized.

The insulator 402 may be formed to contain excess oxygen. Alternatively,oxygen may be added after the insulator 402 is formed. The addition ofoxygen may be performed by an ion implantation method at an accelerationvoltage of higher than or equal to 2 kV and lower than or equal to 100kV and at a dose of greater than or equal to 5×10¹⁴ ions/cm² and lessthan or equal to 5×10¹⁶ ions/cm², for example.

Note that in the case where the insulator 402 is a stacked-layer film,films in the stacked-layer film may be formed using by differentformation methods such as the above formation methods. For example, thefirst film may be formed by a CVD method and the second film may beformed by an ALD method. Alternatively, the first film may be formed bya sputtering method and the second film may be formed by an ALD method.When films are formed by different formation methods as described above,the films can have different functions or different properties.Furthermore, by stacking the films, a more appropriate film can beformed as a stacked-layer film.

In other words, an n-th film (n is a natural number) is formed by atleast one of a sputtering method, a CVD method, an MBE method, a PLDmethod, an ALD method, and the like, and an n+1-th film is formed by atleast one of a sputtering method, a CVD method, an MBE method, a PLDmethod, an ALD method, and the like. Note that the n-th film and then+1-th film may be formed by the same formation method or differentformation methods. Note that the n-th film and the n+2-th film may beformed by the same formation method. Alternatively, all the films may beformed by the same formation method (see FIG. 12B).

Next, the semiconductor 406 a and the semiconductor 406 b are formed inthis order. The semiconductor 406 a and the semiconductor 406 b may beformed by a sputtering method, a CVD method, an MBE method, a PLDmethod, an ALD method, or the like.

In the case where In—Ga—Zn oxide layers are formed as the semiconductor406 a and the semiconductor 406 b by an MOCVD method, trimethylindium,trimethylgallium, dimethylzinc, and the like may be used as the sourcegases. The source gas is not limited to the combination of these gases,triethylindium or the like may be used instead of trimethylindium.Triethylgallium or the like may be used instead of trimethylgallium.Diethylzinc or the like may be used instead of dimethylzinc.

Next, first heat treatment is preferably performed. The first heattreatment is performed at a temperature higher than or equal to 250° C.and lower than or equal to 650° C., preferably higher than or equal to300° C. and lower than or equal to 500° C. The first heat treatment isperformed in an inert gas atmosphere or an atmosphere containing anoxidizing gas at 10 ppm or more, 1% or more, or 10% or more. The firstheat treatment may be performed under a reduced pressure. Alternatively,the first heat treatment may be performed in such a manner that heattreatment is performed in an inert gas atmosphere, and then another heattreatment is performed in an atmosphere containing an oxidizing gas at10 ppm or more, 1% or more, or 10% or more in order to compensatedesorbed oxygen. By the first heat treatment, crystallinity of thesemiconductor 406 a and crystallinity of the semiconductor 406 b can beincreased and impurities such as hydrogen and water can be removed.

Next, a conductor 416 is formed. The conductor 416 may be formed by asputtering method, a CVD method, an MBE method, a PLD method, an ALDmethod, or the like.

The conductor 416 a and the conductor 416 b are formed in such a mannerthat the conductor 416 is formed and then partly etched. Therefore, itis preferable to employ a formation method by which the semiconductor406 b is not damaged when the conductor 416 is formed. In other words,the conductor 416 is preferably formed by an MCVD method or the like.

Note that in the case where the conductor 416 is formed to have astacked-layer structure, films in the stacked-layer film may be formedby different formation methods such as a sputtering method, a CVD method(a plasma CVD method, a thermal CVD method, an MCVD method, an MOCVDmethod, or the like), an MBE method, a PLD method, and an ALD method.For example, the first film may be formed by an MOCVD method and thesecond film may be formed by a sputtering method. Alternatively, thefirst film may be formed by an ALD method and the second film may beformed by an MOCVD method. Alternatively, the first film may be formedby an ALD method and the second film may be formed by a sputteringmethod. Alternatively, the first film may be formed by an ALD method,the second film may be formed by a sputtering method, and the third filmmay be formed by an ALD method. When films are formed by differentformation methods as described above, the films can have differentfunctions or different properties. Furthermore, by stacking the films, amore appropriate film can be formed as a stacked-layer film.

In other words, in the case where the conductor 416 is a stacked-layerfilm, for example, an n-th film (n is a natural number) is formed by atleast one of a sputtering method, a CVD method (a plasma CVD method, athermal CVD method, an MCVD method, an MOCVD method, or the like), anMBE method, a PLD method, an ALD method, and the like and an n+1-th filmis formed by at least one of a sputtering method, a CVD method (a plasmaCVD method, a thermal CVD method, an MCVD method, an MOCVD method, orthe like), an MBE method, a PLD method, an ALD method, and the like.Note that the n-th film and the n+1-th film may be formed by differentformation methods. Note that the n-th film and the n+2-th film may beformed by the same formation method. Alternatively, all the films may beformed by the same formation method.

Note that the conductor 416 or at least one of the films in thestacked-layer film of the conductor 416 and the semiconductor to be thesemiconductor 406 a or the semiconductor to be the semiconductor 406 bmay be formed by the same formation method. For example, both of themmay be formed by an ALD method. Thus, they can be formed withoutexposure to the air. As a result, entry of impurities can be prevented.

Note that the conductor 416 or at least one of the films in thestacked-layer film of the conductor 416, the semiconductor to be thesemiconductor 406 a or the semiconductor to be the semiconductor 406 b,and the insulator 402 or at least one of the films in the stacked-layerfilm of the insulator 402 may be formed by the same formation method.For example, all of them may be formed by a sputtering method. Thus,they can be formed without exposure to the air. As a result, entry ofimpurities can be prevented. Note that a method for manufacturing asemiconductor device of one embodiment of the present invention is notlimited thereto.

Next, a conductor is etched by photolithography and dry etching to formthe conductor 416. Since the conductor 416 becomes the conductor 416 aand the conductor 416 b serving as a source electrode and a drainelectrode of the transistor 492, the conductor 416 preferably has acertain thickness such that the on-state current of the transistor 492is high. Accordingly, the conductor 416 includes a region with athickness of, for example, greater than or equal to 5 nm and less thanor equal to 30 nm, preferably greater than or equal to 5 nm and lessthan or equal to 20 nm, more preferably greater than or equal to 5 nmand less than or equal to 15 nm.

Next, the semiconductor 406 a and the semiconductor 406 b are etchedusing the conductor 416 as a mask, so that the semiconductor 406 a andthe semiconductor 406 b are formed. At this time, when the insulator 402is etched, an s-channel structure is likely to be formed (see FIG. 13).

Next, part of the conductor 416 is etched, so that the conductor 416 aand the conductor 416 b are formed. As described above, the conductor416 formed as a mask for etching the semiconductor 406 a and thesemiconductor 406 b becomes the conductor 416 a and the conductor 416 bserving as the source electrode and the drain electrode of thetransistor 492. Since the conductor 416 to be the conductor 416 a andthe conductor 416 b is also used as a mask, the number of steps formanufacturing the transistor 492 can be reduced. The transistor 492 hasa structure suitable for a miniaturized semiconductor device because thearea occupied by the conductor 416 a and the conductor 416 b can besmall.

Next, a semiconductor to be the semiconductor 406 c is formed. Thesemiconductor to be the semiconductor 406 c can be formed by asputtering method, a CVD method, an MBE method, a PLD method, an ALDmethod, or the like.

In the case where an In—Ga—Zn oxide layer is formed as the semiconductorto be the semiconductor 406 c by an MOCVD method, trimethylindium,trimethylgallium, dimethylzinc, or the like may be used as the sourcegases. The source gas is not limited to the above combination of thesegases, triethylindium or the like may be used instead oftrimethylindium. Triethylgallium or the like may be used instead oftrimethylgallium. Diethylzinc or the like may be used instead ofdimethylzinc.

Next, second heat treatment may be performed. For example, as thesemiconductor 406 a, a semiconductor whose oxygen-transmitting propertyis higher than that of the semiconductor to be the semiconductor 406 cis selected. That is, as the semiconductor to be the semiconductor 406c, a semiconductor whose oxygen-transmitting property is lower than thatof the semiconductor 406 a is selected. In other words, as thesemiconductor 406 a, a semiconductor having a function of passing oxygenis selected. As the semiconductor to be the semiconductor 406 c, asemiconductor having a function of blocking oxygen is selected. In thiscase, by the second heat treatment, excess oxygen in the insulator 402is moved to the semiconductor 406 b through the semiconductor 406 a. Thesemiconductor 406 b is covered with the semiconductor to be thesemiconductor 406 c; thus, outward diffusion of excess oxygen is lesslikely to occur. Therefore, by performing the second heat treatment atthis time, defects (oxygen vacancies) in the semiconductor 406 b can beefficiently reduced. Note that the second heat treatment may beperformed at a temperature such that excess oxygen (oxygen) in theinsulator 402 is diffused to the semiconductor 406 b. For example, thedescription of the first heat treatment may be referred to for thesecond heat treatment. The second heat treatment is preferably performedat a temperature lower than that of the first heat treatment. Thedifference between the temperature of the first heat treatment and thatof the second heat treatment is higher than or equal to 20° C. and lowerthan or equal to 150° C., preferably higher than or equal to 40° C. andlower than or equal to 100° C. Accordingly, superfluous release ofexcess oxygen (oxygen) from the insulator 402 can be inhibited.

Next, an insulator to be the insulator 412 is formed. The insulator tobe the insulator 412 may be formed by a sputtering method, a CVD method,an MBE method, a PLD method, an ALD method, or the like.

Note that in the case where the insulator to be the insulator 412 isformed to have a stacked-layer structure, films in the stacked-layerfilm may be formed by different formation methods such as a sputteringmethod, a CVD method (a plasma CVD method, a thermal CVD method, an MCVDmethod, an MOCVD method, or the like), an MBE method, a PLD method, andan ALD method. For example, the first film may be formed by an MOCVDmethod and the second film may be formed by a sputtering method.Alternatively, the first film may be formed by an ALD method and thesecond film may be formed by an MOCVD method. Alternatively, the firstfilm may be formed by an ALD method and the second film may be formed bya sputtering method. Alternatively, the first film may be formed by anALD method, the second film may be formed by a sputtering method, andthe third film may be formed by an ALD method. Thus, when films areformed by different formation methods, the films can have differentfunctions or different properties. Furthermore, by stacking the films, amore appropriate film can be formed as a stacked-layer film.

In other words, in the case where the insulator to be the insulator 412is a stacked-layer film, for example, an n-th film (n is a naturalnumber) is formed by at least one of a sputtering method, a CVD method(a plasma CVD method, a thermal CVD method, an MCVD method, an MOCVDmethod, or the like), an MBE method, a PLD method, an ALD method, andthe like and an n+1-th film is formed by at least one of a sputteringmethod, a CVD method (a plasma CVD method, a thermal CVD method, an MCVDmethod, an MOCVD method, or the like), an MBE method, a PLD method, anALD method, and the like. Note that the n-th film and the n+1-th filmmay be formed by different formation methods. Note that the n-th filmand the n+2-th film may be formed by the same formation method.Alternatively, all the films may be formed by the same formation method.

Next, third heat treatment may be performed. For example, as thesemiconductor 406 a, a semiconductor whose oxygen-transmitting propertyis higher than that of the semiconductor to be the semiconductor 406 cis selected. That is, as the semiconductor to be the semiconductor 406c, a semiconductor whose oxygen-transmitting property is lower than thatof the semiconductor 406 a. As the semiconductor to be the semiconductor406 c, a semiconductor having a function of blocking oxygen is selected.For example, as the semiconductor 406 a, a semiconductor whoseoxygen-transmitting property is higher than that of the insulator to bethe insulator 412 is selected. That is, as the insulator to be theinsulator 412, a semiconductor whose oxygen-transmitting property islower than that of the semiconductor 406 a is selected. In other words,as the semiconductor 406 a, a semiconductor having a function of passingoxygen is selected. As the insulator to be the insulator 412, aninsulator having a function of blocking oxygen is selected. In thiscase, by the third heat treatment, excess oxygen in the insulator 402 ismoved to the semiconductor 406 b through the semiconductor 406 a. Thesemiconductor 406 b is covered with the semiconductor to be thesemiconductor 406 c and the insulator to be the insulator 412; thus,outward diffusion of excess oxygen is less likely to occur. Therefore,by performing the third heat treatment at this time, defects (oxygenvacancies) in the semiconductor 406 b can be efficiently reduced. Notethat the third heat treatment may be performed at a temperature suchthat excess oxygen (oxygen) in the insulator 402 is diffused to thesemiconductor 406 b. For example, the description of the first heattreatment may be referred to for the third heat treatment. The thirdheat treatment is preferably performed at a temperature lower than thatof the first heat treatment. The difference between the temperature ofthe first heat treatment and that of the third heat treatment is higherthan or equal to 20° C. and lower than or equal to 150° C., preferablyhigher than or equal to 40° C. and lower than or equal to 100° C.Accordingly, superfluous release of excess oxygen (oxygen) from theinsulator 402 can be inhibited. Note that in the case where theinsulator to be the insulator 412 has a function of blocking oxygen, thesemiconductor to be the semiconductor 406 c does not necessarily have afunction of blocking oxygen.

Next, a conductor to be the conductor 404 is formed. The conductor to bethe conductor 404 may be formed by a sputtering method, a CVD method, anMBE method, a PLD method, an ALD method, or the like.

The insulator to be the insulator 412 functions as a gate insulator ofthe transistor 492. Therefore, the conductor to be the conductor 404 ispreferably formed by a formation method by which the insulator to be theinsulator 412 is not damaged when the conductor to be the conductor 404is formed. In other words, the conductor is preferably formed by an MCVDmethod or the like.

Note that in the case where the conductor to be the conductor 404 isformed to have a stacked-layer structure, films in the stacked-layerfilm may be formed by different formation methods such as a sputteringmethod, a CVD method (a plasma CVD method, a thermal CVD method, an MCVDmethod, an MOCVD method, or the like), an MBE method, a PLD method, andan ALD method. For example, the first film may be formed by an MOCVDmethod and the second film may be formed by a sputtering method.Alternatively, the first film may be formed by an ALD method and thesecond film may be formed by an MOCVD method. Alternatively, the firstfilm may be formed by an ALD method and the second film may be formed bya sputtering method. Alternatively, the first film may be formed by anALD method, the second film may be formed by a sputtering method, andthe third film may be formed by an ALD method. Thus, when films areformed by different formation methods, the films can have differentfunctions or different properties. Furthermore, by stacking the films, amore appropriate film can be formed as a stacked-layer film.

In other words, in the case where the conductor to be the conductor 404is a stacked-layer film, for example, an n-th film (n is a naturalnumber) is formed by at least one of a sputtering method, a CVD method(a plasma CVD method, a thermal CVD method, an MCVD method, an MOCVDmethod, or the like), an MBE method, a PLD method, an ALD method, andthe like and an n+1-th film is formed by at least one of a sputteringmethod, a CVD method (a plasma CVD method, a thermal CVD method, an MCVDmethod, an MOCVD method, or the like), an MBE method, a PLD method, anALD method, and the like. Note that the n-th film and the n+1-th filmmay be formed by different formation methods. Note that the n-th filmand the n+2-th film may be formed by the same formation method.Alternatively, all the films may be formed by the same formation method.

Note that the conductor to be the conductor 404 or at least one of thefilms in the stacked-layer film of the conductor to be the conductor 404and the insulator to be the insulator 412 or at least one of the filmsin the stacked-layer film of the insulator to be the insulator 412 maybe formed by the same formation method. For example, both of them may beformed by an ALD method. Thus, they can be formed without exposure tothe air. As a result, entry of impurities can be prevented. For example,the conductor to be the conductor 404 and the insulator to be theinsulator 412 which are in contact with each other may be formed by thesame formation method. Thus, the formation can be performed in the samechamber. As a result, entry of impurities can be prevented.

Note that the conductor to be the conductor 404 or at least one of thefilms in the stacked-layer film of the conductor to be the conductor 404and the insulator to be the insulator 412 or at least one of the filmsin the stacked-layer film of the insulator to be the insulator 412 maybe formed by the same formation method. For example, all of them may beformed by a sputtering method. Thus, they can be formed without exposureto the air. As a result, entry of impurities can be prevented.

Next, the conductor to be the conductor 404 is partly etched, so thatthe conductor 404 is formed. The conductor 404 is formed to overlap withat least part of the semiconductor 406 b.

Next, in a manner similar to that of the conductor to be the conductor404, the insulator to be the insulator 412 is partly etched, so that theinsulator 412 is formed.

Next, in a manner similar to those of the conductor to be the conductor404 and the insulator to be the insulator 412, the semiconductor to bethe semiconductor 406 c is partly etched, so that the semiconductor 406c is formed.

The conductor to be the conductor 404, the insulator to be the insulator412, and the semiconductor to be the semiconductor 406 c may be partlyetched through the same photolithography process, for example.Alternatively, the insulator to be the insulator 412 and thesemiconductor to be the semiconductor 406 c may be etched using theconductor 404 as a mask. Thus, the conductor 404, the insulator 412, andthe semiconductor 406 c have similar shapes in the top view. Theinsulator 412 and/or the semiconductor 406 c may project as comparedwith the conductor 404 or the conductor 404 may project as compared withthe insulator 412 and/or the semiconductor 406 c. With such a shape,shape defects are reduced and gate leakage current can be reduced insome cases.

Next, the insulator 408 is formed. The insulator 408 may be formed by asputtering method, a CVD method, an MBE method, a PLD method, an ALDmethod, or the like.

Next, fourth heat treatment may be performed. For example, as thesemiconductor 406 a, a semiconductor whose oxygen-transmitting propertyis higher than that of the semiconductor 406 c is selected. In otherwords, as the semiconductor 406 c, a semiconductor whoseoxygen-transmitting property is lower than that of the semiconductor 406a is selected. As the semiconductor 406 c, a semiconductor having afunction of blocking oxygen is selected. For example, as thesemiconductor 406 a, a semiconductor whose oxygen-transmitting propertyis higher than that of the insulator 412 is selected. In other words, asthe insulator 412, a semiconductor whose oxygen-transmitting property islower than that of the semiconductor 406 a is selected. For example, asthe semiconductor 406 a, a semiconductor whose oxygen-transmittingproperty is higher than that of the insulator 408 is selected. That is,as the insulator 408, a semiconductor whose oxygen-transmitting propertyis lower than that of the semiconductor 406 a is selected. In otherwords, as the semiconductor 406 a, a semiconductor having a function ofpassing oxygen is selected. As the insulator 408, an insulator having afunction of blocking oxygen is selected. In this case, by the fourthheat treatment, excess oxygen in the insulator 402 is moved to thesemiconductor 406 b through the semiconductor 406 a. The semiconductor406 b is covered with any of the semiconductor 406 c, the insulator 412,and the insulator 408; thus, outward diffusion of excess oxygen is lesslikely to occur. Therefore, by performing the fourth heat treatment atthis time, defects (oxygen vacancies) in the semiconductor 406 b can beefficiently reduced. Note that the fourth heat treatment may beperformed at a temperature such that excess oxygen (oxygen) in theinsulator 402 is diffused to the semiconductor 406 b. For example, thedescription of the first heat treatment may be referred to for thefourth heat treatment. The fourth heat treatment is preferably performedat a temperature lower than that of the first heat treatment. Thedifference between the temperature of the first heat treatment and thatof the fourth heat treatment is higher than or equal to 20° C. and lowerthan or equal to 150° C., preferably higher than or equal to 40° C. andlower than or equal to 100° C. Accordingly, superfluous release ofexcess oxygen (oxygen) from the insulator 402 can be inhibited. Notethat in the case where the insulator 408 has a function of blockingoxygen, the semiconductor 406 c and/or the insulator 412 does notnecessarily have a function of blocking oxygen.

One or more of the first heat treatment, the second heat treatment, thethird heat treatment, and the fourth heat treatment are not necessarilyperformed.

Next, the insulator 418 is formed. The insulator 418 may be formed by asputtering method, a CVD method, an MBE method, a PLD method, an ALDmethod, or the like (see FIG. 14).

A contact hole is formed by photolithography and dry etching. Thecontact hole penetrates the insulator 418, the insulator 408, theinsulator 412, the semiconductor 406 c, the conductor 416 a, thesemiconductor 406 b, the semiconductor 406 a, the insulator 402, theinsulator 401, the insulator 484, the conductor 417, and the insulator480, and reaches a top surface of the conductor 454. At the same time,another contact hole is formed. The contact hole penetrates theinsulator 418, the insulator 408, the insulator 412, the semiconductor406 c, the conductor 416 b, the semiconductor 406 b, the semiconductor406 a, the insulator 402, the insulator 401, the insulator 484, theinsulator 482, and the insulator 480, and reaches a top surface of theconductor 474. Forming the contact holes at the same time can reduce thenumbers of masks and manufacturing steps. Depending on a circuitconfiguration, it is favorable to form the contact holes separately withdifferent masks in some cases because charge-up damage in the contacthole due to the dry etching can be suppressed. Although not illustrated,a contact hole may be formed by photolithography and dry etching foreach insulator and each conductor. In addition, the contact holes ininsulators and conductors are not necessarily formed linearly as long asthe elements (the transistors and the capacitor) are electricallyconnected to one another with the conductors in the contact holes (seeFIG. 15).

Next, the capacitor 493 is formed. The insulator 419 is formed on a sidesurface of the contact hole that penetrates the conductor 417 byoxidizing the side surface of the contact hole. As the oxidation method,a thermal oxidation method or a radical oxidation method can be used,for example. When the thickness of the insulator and the thickness ofthe oxide film are constant, the capacitance value can be controlled bycontrolling the diameter of the contact hole. When the diameter of thecontact hole and the thickness of the oxide film are constant, thecapacitance value of the capacitor 493 can be controlled by controllingthe thickness of the insulator 482. To obtain a higher capacitance, thethickness of the insulator 482 may be increased. By controlling thecapacitance value of the capacitor 493, a highly integratedsemiconductor device can be provided. Alternatively, a semiconductordevice with a large storage capacity with respect to an area occupied bythe capacitor 493 can be provided (see FIG. 16).

Next, the contact hole is filled with the conductor 421 and CMPtreatment is performed to remove the conductor over the insulator 418.The conductor is formed by, for example, a sputtering method, a CVDmethod, or an ALD method. Although CMP treatment is used here to removethe conductor over the insulator 418, other treatment may be employed.Alternatively, the CMP treatment may be combined with etching (dryetching or wet etching) treatment or plasma treatment. Then, theconductor 424 is deposited on the insulator 418 and the conductor 424 aand 424 b are formed by photolithography and dry etching. The conductor424 is formed by a sputtering method, a CVD method, an ALD method, orthe like (see FIG. 17). Through the above steps, the semiconductordevice illustrated in FIG. 1 can be manufactured.

The capacitor can be formed over the transistor 492 (see FIG. 3).Alternatively, capacitors can be formed over and under the transistor492 (see FIG. 4).

<Semiconductor Device>

An example of a semiconductor device of one embodiment of the presentinvention is shown below.

A circuit diagram in FIG. 18A shows a configuration of a so-called CMOScircuit in which the p-channel transistor 2200 and the n-channeltransistor 2100 are connected to each other in series and in which gatesof them are connected to each other.

A circuit diagram in FIG. 18B shows a configuration in which sources ofthe transistors 2100 and 2200 are connected to each other and drains ofthe transistors 2100 and 2200 are connected to each other. With such aconfiguration, the transistors can function as an analog switch.

For example, as the transistor 2100, the transistor 492 or the like maybe used. For example, as the transistor 2200, the transistor 491 or thelike may be used. An example of a semiconductor device (memory device)which can retain stored data even when not powered and which has anunlimited number of write cycles is shown in FIGS. 19A and 19B.

The semiconductor device illustrated in FIG. 19A includes a transistor3200 using a first semiconductor, a transistor 3300 using a secondsemiconductor, and a capacitor 3400. Note that the transistor 492 or thelike may be used as the transistor 3300. As the transistor 3200, thetransistor 491 or the like may be used. As the capacitor 3400, thecapacitor 493, the capacitor 494, or the like may be used.

In the case where the transistor 3300 is a transistor using an oxidesemiconductor, since the off-state current of the transistor 3300 islow, stored data can be retained for a long period at a predeterminednode of the semiconductor device. In other words, power consumption ofthe semiconductor device can be reduced because refresh operationbecomes unnecessary or the frequency of refresh operation can beextremely low.

In FIG. 19A, a first wiring 3001 is electrically connected to a sourceof the transistor 3200. A second wiring 3002 is electrically connectedto a drain of the transistor 3200. A third wiring 3003 is electricallyconnected to one of the source and the drain of the transistor 3300. Afourth wiring 3004 is electrically connected to the gate of thetransistor 3300. The gate of the transistor 3200 and the other of thesource and the drain of the transistor 3300 are electrically connectedto a first terminal of the capacitor 3400. A fifth wiring 3005 iselectrically connected to a second terminal of the capacitor 3400.

The semiconductor device in FIG. 19A has a feature that the potential ofthe gate of the transistor 3200 can be retained, and thus enableswriting, retaining, and reading of data as follows.

Writing and retaining of data are described. First, the potential of thefourth wiring 3004 is set to a potential at which the transistor 3300 isturned on, so that the transistor 3300 is turned on. Accordingly, thepotential of the third wiring 3003 is supplied to a node FG where thegate of the transistor 3200 and the first terminal of the capacitor 3400are electrically connected to each other. That is, a predeterminedcharge is supplied to the gate of the transistor 3200 (writing). Here,one of two kinds of charges providing different potential levels(hereinafter referred to as a low-level charge and a high-level charge)is supplied. After that, the potential of the fourth wiring 3004 is setto a potential at which the transistor 3300 is turned off. Thus, thecharge is held at the node FG (retaining).

Since the off-state current of the transistor 3300 is extremely low, thecharge of the node FG is retained for a long time.

Next, reading of data is described. An appropriate potential (a readingpotential) is supplied to the fifth wiring 3005 while a predeterminedpotential (a constant potential) is supplied to the first wiring 3001,whereby the potential of the second wiring 3002 varies depending on theamount of charge retained in the node FG. This is because in the case ofusing an n-channel transistor as the transistor 3200, an apparentthreshold voltage V_(th) _(—) _(H) at the time when the high-levelcharge is given to the gate of the transistor 3200 is lower than anapparent threshold voltage V_(th) _(—) _(L) at the time when thelow-level charge is given to the gate of the transistor 3200. Here, anapparent threshold voltage refers to the potential of the fifth wiring3005 which is needed to turn on the transistor 3200. Thus, the potentialof the fifth wiring 3005 is set to a potential V₀ which is betweenV_(th) _(—) _(H) and V_(th) _(—) _(L), whereby charge supplied to thenode FG can be determined. For example, in the case where the high-levelcharge is supplied to the node FG in writing and the potential of thefifth wiring 3005 is V₀ (>V_(th) _(—) _(H)), the transistor 3200 isturned on. On the other hand, in the case where the low-level charge issupplied to the node FG in writing, even when the potential of the fifthwiring 3005 is V₀ (<V_(th) _(—) _(L)), the transistor 3200 remains off.Thus, the data retained in the node FG can be read by determining thepotential of the second wiring 3002.

Note that in the case where memory cells are arrayed, it is necessarythat data of a desired memory cell is read in read operation. In thecase where data of the other memory cells is not read, the fifth wiring3005 may be supplied with a potential at which the transistor 3200 isturned off regardless of the charge supplied to the node FG, that is, apotential lower than V_(th) _(—) _(H). Alternatively, the fifth wiring3005 may be supplied with a potential at which the transistor 3200 isturned on regardless of the charge supplied to the node FG, that is, apotential higher than V_(th) _(—) _(L).

The semiconductor device in FIG. 19B is different form the semiconductordevice in FIG. 19A in that the transistor 3200 is not provided. Also inthis case, writing and retaining operation of data can be performed in amanner similar to that of the semiconductor device in FIG. 19A.

Reading of data in the semiconductor device in FIG. 19B is described.When the transistor 3300 is turned on, the third wiring 3003 which is ina floating state and the capacitor 3400 are electrically connected toeach other, and the charge is redistributed between the third wiring3003 and the capacitor 3400. As a result, the potential of the thirdwiring 3003 is changed. The amount of change in potential of the thirdwiring 3003 varies depending on the potential of the first terminal ofthe capacitor 3400 (or the charge accumulated in the capacitor 3400).

For example, the potential of the third wiring 3003 after the chargeredistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potentialof the first terminal of the capacitor 3400, C is the capacitance of thecapacitor 3400, C_(B) is the capacitance component of the third wiring3003, and V_(B0) is the potential of the third wiring 3003 before thecharge redistribution. Thus, it can be found that, assuming that thememory cell is in either of two states in which the potential of thefirst terminal of the capacitor 3400 is V₁ and V₀ (V₁>V₀), the potentialof the third wiring 3003 in the case of retaining the potential V₁(=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher than the potential of thethird wiring 3003 in the case of retaining the potential V₀(=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the third wiring 3003 with apredetermined potential, data can be read.

In this case, a transistor including the first semiconductor may be usedfor a driver circuit for driving a memory cell, and a transistorincluding the second semiconductor may be stacked over the drivercircuit as the transistor 3300.

When including a transistor using an oxide semiconductor and having anextremely low off-state current, the semiconductor device describedabove can retain stored data for a long time. In other words, refreshoperation becomes unnecessary or the frequency of the refresh operationcan be extremely low, which leads to a sufficient reduction in powerconsumption. Moreover, stored data can be retained for a long time evenwhen power is not supplied (note that a potential is preferably fixed).

In the semiconductor device, high voltage is not needed for writing dataand deterioration of elements is less likely to occur. Unlike in aconventional nonvolatile memory, for example, it is not necessary toinject and extract electrons into and from a floating gate; thus, aproblem such as deterioration of an insulator is not caused. That is,the semiconductor device of one embodiment of the present invention doesnot have a limit on the number of times data can be rewritten, which isa problem of a conventional nonvolatile memory, and the reliabilitythereof is drastically improved. Furthermore, data is written dependingon the state of the transistor (on or off), whereby high-speed operationcan be easily achieved.

<RFIC Tag>

An RFIC tag including the transistor or the memory device is describedbelow with reference to FIG. 20.

The RFIC tag of one embodiment of the present invention includes amemory circuit, stores data in the memory circuit, and transmits andreceives data to/from the outside by using contactless means, forexample, wireless communication. With these features, the RFIC tag canbe used for an individual authentication system in which an object orthe like is recognized by reading the individual information, forexample. Note that the RFIC tag is required to have high reliability inorder to be used for this purpose.

A configuration of the RFIC tag will be described with reference to FIG.20. FIG. 20 is a block diagram illustrating a configuration example ofan RFIC tag.

As shown in FIG. 20, an RFIC tag 800 includes an antenna 804 whichreceives a radio signal 803 that is transmitted from an antenna 802connected to a communication device 801 (also referred to as aninterrogator, a reader/writer, or the like). The RFIC tag 800 includes arectifier circuit 805, a constant voltage circuit 806, a demodulationcircuit 807, a modulation circuit 808, a logic circuit 809, a memorycircuit 810, and a ROM 811. A semiconductor of a transistor having arectifying function included in the demodulation circuit 807 may be amaterial which enables a reverse current to be low enough, for example,an oxide semiconductor. This can suppress the phenomenon of a rectifyingfunction becoming weaker due to generation of a reverse current andprevent saturation of the output from the demodulation circuit. In otherwords, the input to the demodulation circuit and the output from thedemodulation circuit can have a relation closer to a linear relation.Note that data transmission methods are roughly classified into thefollowing three methods: an electromagnetic coupling method in which apair of coils is provided so as to face each other and communicates witheach other by mutual induction, an electromagnetic induction method inwhich communication is performed using an induction field, and a radiowave method in which communication is performed using a radio wave. Anyof these methods can be used in the RFIC tag 800.

Next, the structure of each circuit will be described. The antenna 804exchanges the radio signal 803 with the antenna 802 which is connectedto the communication device 801. The rectifier circuit 805 generates aninput potential by rectification, for example, half-wave voltage doublerrectification of an input alternating signal generated by reception of aradio signal at the antenna 804 and smoothing of the rectified signalwith a capacitor provided in a later stage in the rectifier circuit 805.Note that a limiter circuit may be provided on an input side or anoutput side of the rectifier circuit 805. The limiter circuit controlselectric power so that electric power which is higher than or equal tocertain electric power is not input to a circuit in a later stage if theamplitude of the input alternating signal is high and an internalgeneration voltage is high.

The constant voltage circuit 806 generates a stable power supply voltagefrom an input potential and supplies it to each circuit. Note that theconstant voltage circuit 806 may include a reset signal generationcircuit. The reset signal generation circuit is a circuit whichgenerates a reset signal of the logic circuit 809 by utilizing rise ofthe stable power supply voltage.

The demodulation circuit 807 demodulates the input alternating signal byenvelope detection and generates the demodulated signal. Furthermore,the modulation circuit 808 performs modulation in accordance with datato be output from the antenna 804.

The logic circuit 809 analyzes and processes the demodulated signal. Thememory circuit 810 holds the input data and includes a row decoder, acolumn decoder, a memory region, and the like. Furthermore, the ROM 811stores an identification number (ID) or the like and outputs it inaccordance with processing.

Note that the decision whether each circuit described above is providedor not can be made as appropriate as needed.

Here, the above-described memory device can be used as the memorycircuit 810. Since the memory device of one embodiment of the presentinvention can retain data even when not powered, the memory device issuitable for an RFIC tag. Furthermore, the memory device of oneembodiment of the present invention needs power (voltage) needed fordata writing lower than that needed in a conventional nonvolatilememory; thus, it is possible to prevent a difference between the maximumcommunication range in data reading and that in data writing. Inaddition, it is possible to suppress malfunction or incorrect writingwhich is caused by power shortage in data writing.

Since the memory device of one embodiment of the present invention canbe used as a nonvolatile memory, it can also be used as the ROM 811. Inthis case, it is preferable that a manufacturer separately prepare acommand for writing data to the ROM 811 so that a user cannot rewritedata freely. Since the manufacturer gives identification numbers beforeshipment and then starts shipment of products, instead of puttingidentification numbers to all the manufactured RFIC tags, it is possibleto put identification numbers to only good products to be shipped. Thus,the identification numbers of the shipped products are in series andcustomer management corresponding to the shipped products is easilyperformed.

<Application Examples of RFIC Tag>

Application examples of the RFIC tag of one embodiment of the presentinvention are shown below with reference to FIGS. 21A to 21F. The RFICtag is widely used and can be provided for, for example, products suchas bills, coins, securities, bearer bonds, documents (e.g., driver'slicenses or resident's cards, see FIG. 21A), packaging containers (e.g.,wrapping paper or bottles, see FIG. 21C), recording media (e.g., DVDs orvideo tapes, see FIG. 21B), vehicles (e.g., bicycles, see FIG. 21D),personal belongings (e.g., bags or glasses), foods, plants, animals,human bodies, clothing, household goods, medical supplies such asmedicine and chemicals, and electronic devices (e.g., liquid crystaldisplay devices, EL display devices, television sets, or cellularphones), or tags on products (see FIGS. 21E and 21F).

An RFIC tag 4000 of one embodiment of the present invention is fixed onproducts by, for example, being attached to a surface thereof or beingembedded therein. For example, the RFIC tag 4000 is fixed to eachproduct by being embedded in paper of a book, or embedded in an organicresin of a package. The RFIC tag 4000 of one embodiment of the presentinvention is small, thin, and lightweight, so that the design of aproduct is not impaired even after the RFIC tag 4000 of one embodimentof the present invention is fixed thereto. Furthermore, bills, coins,securities, bearer bonds, documents, or the like can have identificationfunctions by being provided with the RFIC tag 4000 of one embodiment ofthe present invention, and the identification functions can be utilizedto prevent counterfeits. Moreover, the efficiency of a system such as aninspection system can be improved by providing the RFIC tag 4000 of oneembodiment of the present invention for packaging containers, recordingmedia, personal belongings, foods, clothing, household goods, electronicdevices, or the like. Vehicles can also have higher security againsttheft or the like by being provided with the RFIC tag 4000 of oneembodiment of the present invention.

As described above, the RFIC tag of one embodiment of the presentinvention can be used for the above-described purposes.

<CPU>

A CPU including a semiconductor device such as any of theabove-described transistors or the above-described memory device isdescribed below.

FIG. 22 is a block diagram illustrating a configuration example of a CPUincluding any of the above-described transistors as a component.

The CPU illustrated in FIG. 22 includes, over a substrate 1190, anarithmetic logic unit (ALU) 1191, an ALU controller 1192, an instructiondecoder 1193, an interrupt controller 1194, a timing controller 1195, aregister 1196, a register controller 1197, a bus interface (Bus I/F)1198, a rewritable ROM 1199, and a ROM interface (ROM I/F) 1189. Asemiconductor substrate, an SOI substrate, a glass substrate, or thelike is used as the substrate 1190. The ROM 1199 and the ROM interface1189 may be provided over a separate chip. Needless to say, the CPU inFIG. 22 is just an example in which the configuration has beensimplified, and an actual CPU may have a variety of configurationsdepending on the application. For example, the CPU may have thefollowing configuration: a structure including the CPU illustrated inFIG. 22 or an arithmetic circuit is considered as one core; a pluralityof the cores are included; and the cores operate in parallel. The numberof bits that the CPU can process in an internal arithmetic circuit or ina data bus can be 8, 16, 32, or 64, for example.

An instruction that is input to the CPU through the bus interface 1198is input to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 judges an interrupt request from an external input/output device ora peripheral circuit on the basis of its priority or a mask state, andprocesses the request. The register controller 1197 generates an addressof the register 1196, and reads/writes data from/to the register 1196 inaccordance with the state of the CPU.

The timing controller 1195 generates signals for controlling operationtimings of the ALU 1191, the ALU controller 1192, the instructiondecoder 1193, the interrupt controller 1194, and the register controller1197. For example, the timing controller 1195 includes an internal clockgenerator for generating an internal clock signal CLK2 based on areference clock signal CLK1, and supplies the internal clock signal CLK2to the above circuits.

In the CPU illustrated in FIG. 22, a memory cell is provided in theregister 1196. For the memory cell of the register 1196, any of theabove-described transistors, the above-described memory device, or thelike can be used.

In the CPU illustrated in FIG. 22, the register controller 1197 selectsoperation of retaining data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, the register controller 1197selects whether data is retained by a flip-flop or by a capacitor in thememory cell included in the register 1196. When data retaining by theflip-flop is selected, a power supply voltage is supplied to the memorycell in the register 1196. When data retaining by the capacitor isselected, the data is rewritten in the capacitor, and supply of powersupply voltage to the memory cell in the register 1196 can be stopped.

FIG. 23 is an example of a circuit diagram of a memory element that canbe used as the register 1196. A memory element 1200 includes a circuit1201 in which stored data is volatile when power supply is stopped, acircuit 1202 in which stored data is nonvolatile even when power supplyis stopped, a switch 1203, a switch 1204, a logic element 1206, acapacitor 1207, and a circuit 1220 having a selecting function. Thecircuit 1202 includes a capacitor 1208, a transistor 1209, and atransistor 1210. Note that the memory element 1200 may further includeanother element such as a diode, a resistor, or an inductor, as needed.

Here, the above-described memory device can be used as the circuit 1202.When supply of a power supply voltage to the memory element 1200 isstopped, GND (0 V) or a potential at which the transistor 1209 in thecircuit 1202 is turned off continues to be input to a gate of thetransistor 1209. For example, the gate of the transistor 1209 isgrounded through a load such as a resistor.

Shown here is an example in which the switch 1203 is a transistor 1213having one conductivity type (e.g., an n-channel transistor) and theswitch 1204 is a transistor 1214 having a conductivity type opposite tothe one conductivity type (e.g., a p-channel transistor). A firstterminal of the switch 1203 corresponds to one of a source and a drainof the transistor 1213, a second terminal of the switch 1203 correspondsto the other of the source and the drain of the transistor 1213, andconduction or non-conduction between the first terminal and the secondterminal of the switch 1203 (i.e., the on/off state of the transistor1213) is selected by a control signal RD input to a gate of thetransistor 1213. A first terminal of the switch 1204 corresponds to oneof a source and a drain of the transistor 1214, a second terminal of theswitch 1204 corresponds to the other of the source and the drain of thetransistor 1214, and conduction or non-conduction between the firstterminal and the second terminal of the switch 1204 (i.e., the on/offstate of the transistor 1214) is selected by the control signal RD inputto a gate of the transistor 1214.

One of a source and a drain of the transistor 1209 is electricallyconnected to one of a pair of electrodes of the capacitor 1208 and agate of the transistor 1210. Here, the connection portion is referred toas a node M2. One of a source and a drain of the transistor 1210 iselectrically connected to a line which can supply a low power supplypotential (e.g., a GND line), and the other thereof is electricallyconnected to the first terminal of the switch 1203 (the one of thesource and the drain of the transistor 1213). The second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is electrically connected to the first terminal of the switch 1204(the one of the source and the drain of the transistor 1214). The secondterminal of the switch 1204 (the other of the source and the drain ofthe transistor 1214) is electrically connected to a line which cansupply a power supply potential VDD. The second terminal of the switch1203 (the other of the source and the drain of the transistor 1213), thefirst terminal of the switch 1204 (the one of the source and the drainof the transistor 1214), an input terminal of the logic element 1206,and one of a pair of electrodes of the capacitor 1207 are electricallyconnected to each other. Here, the connection portion is referred to asa node M1. The other of the pair of electrodes of the capacitor 1207 canbe supplied with a constant potential. For example, the other of thepair of electrodes of the capacitor 1207 can be supplied with a lowpower supply potential (e.g., GND) or a high power supply potential(e.g., VDD). The other of the pair of electrodes of the capacitor 1207is electrically connected to the line which can supply a low powersupply potential (e.g., a GND line). The other of the pair of electrodesof the capacitor 1208 can be supplied with a constant potential. Forexample, the other of the pair of electrodes of the capacitor 1208 canbe supplied with the low power supply potential (e.g., GND) or the highpower supply potential (e.g., VDD). The other of the pair of electrodesof the capacitor 1208 is electrically connected to the line which cansupply a low power supply potential (e.g., a GND line).

The capacitor 1207 and the capacitor 1208 are not necessarily providedas long as the parasitic capacitance of the transistor, the wiring, orthe like is actively utilized.

A control signal WE is input to a first gate (a first gate electrode) ofthe transistor 1209. As for each of the switch 1203 and the switch 1204,a conduction state or a non-conduction state between the first terminaland the second terminal is selected by the control signal RD which isdifferent from the control signal WE. When the first terminal and thesecond terminal of one of the switches are in the conduction state, thefirst terminal and the second terminal of the other of the switches arein the non-conduction state.

A signal corresponding to data retained in the circuit 1201 is input tothe other of the source and the drain of the transistor 1209. FIG. 23illustrates an example in which a signal output from the circuit 1201 isinput to the other of the source and the drain of the transistor 1209.The logic value of a signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is inverted by the logic element 1206, and the inverted signal isinput to the circuit 1201 through the circuit 1220.

In the example of FIG. 23, a signal output from the second terminal ofthe switch 1203 (the other of the source and the drain of the transistor1213) is input to the circuit 1201 through the logic element 1206 andthe circuit 1220; however, one embodiment of the present invention isnot limited thereto. The signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) may be input to the circuit 1201 without its logic value beinginverted. For example, in the case where the circuit 1201 includes anode in which a signal obtained by inversion of the logic value of asignal input from the input terminal is retained, the signal output fromthe second terminal of the switch 1203 (the other of the source and thedrain of the transistor 1213) can be input to the node.

In FIG. 23, among the transistors used in the memory element 1200, thetransistor 492 or the like may be used as the transistor 1209, forexample. As the transistors other than the transistor 1209, thetransistor 491 or the like may be used, for example.

As the circuit 1201 in FIG. 23, for example, a flip-flop circuit can beused. As the logic element 1206, for example, an inverter or a clockedinverter can be used.

In a period during which the memory element 1200 is not supplied withthe power supply voltage, the semiconductor device of one embodiment ofthe present invention can retain data stored in the circuit 1201 by thecapacitor 1208 which is provided in the circuit 1202.

The off-state current of a transistor in which a channel is formed in anoxide semiconductor is extremely low. For example, the off-state currentof a transistor in which a channel is formed in an oxide semiconductoris significantly lower than that of a transistor in which a channel isformed in silicon having crystallinity. Thus, when the transistor isused as the transistor 1209, a signal held in the capacitor 1208 isretained for a long time also in a period during which the power supplyvoltage is not supplied to the memory element 1200. The memory element1200 can accordingly retain the stored content (data) also in a periodduring which the supply of the power supply voltage is stopped.

Since the memory element performs pre-charge operation with the switch1203 and the switch 1204, the time required for the circuit 1201 toretain original data again after the supply of the power supply voltageis restarted can be shortened.

In the circuit 1202, a signal retained by the capacitor 1208 is input tothe gate of the transistor 1210. Therefore, after supply of the powersupply voltage to the memory element 1200 is restarted, the signalretained by the capacitor 1208 can be converted into the onecorresponding to the state (the on state or the off state) of thetransistor 1210 to be read from the circuit 1202. Consequently, anoriginal signal can be accurately read even when a potentialcorresponding to the signal retained by the capacitor 1208 varies tosome degree.

By applying the above-described memory element 1200 to a memory devicesuch as a register or a cache memory included in a processor, data inthe memory device can be prevented from being lost owing to the stop ofthe supply of the power supply voltage. Furthermore, shortly after thesupply of the power supply voltage is restarted, the memory device canbe returned to the same state as that before the power supply isstopped. Therefore, the power supply can be stopped even for a shorttime in the processor or one or a plurality of logic circuits includedin the processor, resulting in lower power consumption.

Although the memory element 1200 is used in a CPU, the memory element1200 can also be used in an LSI such as a digital signal processor(DSP), a custom LSI, or a programmable logic device (PLD), and a radiofrequency integrated circuit (RF-IC).

<Display Device>

The following shows configuration examples of a display device of oneembodiment of the present invention.

Configuration Example

FIG. 24A is a top view of a display device of one embodiment of thepresent invention. FIG. 24B illustrates a pixel circuit where a liquidcrystal element is used for a pixel of a display device of oneembodiment of the present invention. FIG. 24C illustrates a pixelcircuit where an organic EL element is used for a pixel of a displaydevice of one embodiment of the present invention.

The transistor 492 or the like can be used as a transistor used for thepixel. Here, an example in which an n-channel transistor is used isshown. Note that a transistor manufactured through the same steps as thetransistor used for the pixel may be used for a driver circuit. Thus, byusing the above-described transistor for a pixel or a driver circuit,the display device can have high display quality and/or highreliability.

FIG. 24A illustrates an example of a top view of an active matrixdisplay device. A pixel portion 5001, a first scan line driver circuit5002, a second scan line driver circuit 5003, and a signal line drivercircuit 5004 are provided over a substrate 5000 in the display device.The pixel portion 5001 is electrically connected to the signal linedriver circuit 5004 through a plurality of signal lines and iselectrically connected to the first scan line driver circuit 5002 andthe second scan line driver circuit 5003 through a plurality of scanlines. Pixels including display elements are provided in respectiveregions divided by the scan lines and the signal lines. The substrate5000 of the display device is electrically connected to a timing controlcircuit (also referred to as a controller or a control IC) through aconnection portion such as a flexible printed circuit (FPC).

The first scan line driver circuit 5002, the second scan line drivercircuit 5003, and the signal line driver circuit 5004 are formed overthe substrate 5000 where the pixel portion 5001 is formed. Therefore, adisplay device can be manufactured at cost lower than that in the casewhere a driver circuit is separately formed. Furthermore, in the casewhere a driver circuit is separately formed, the number of wiringconnections is increased. By providing the driver circuit over thesubstrate 5000, the number of wiring connections can be reduced.Accordingly, the reliability and/or yield can be improved.

[Liquid Crystal Display Device]

FIG. 24B illustrates an example of a circuit configuration of the pixel.Here, a pixel circuit which is applicable to a pixel of a VA liquidcrystal display device, or the like is illustrated.

This pixel circuit can be applied to a structure in which one pixelincludes a plurality of pixel electrodes. The pixel electrodes areconnected to different transistors, and the transistors can be drivenwith different gate signals. Accordingly, signals applied to individualpixel electrodes in a multi-domain pixel can be controlledindependently.

A gate wiring 5012 of a transistor 5016 and a gate wiring 5013 of atransistor 5017 are separated so that different gate signals can besupplied thereto. In contrast, a source or drain electrode 5014functioning as a data line is shared by the transistors 5016 and 5017.Any of the above-described transistors can be used as appropriate aseach of the transistors 5016 and 5017. Thus, the liquid crystal displaydevice can have high display quality and/or high reliability.

A first pixel electrode is electrically connected to the transistor 5016and a second pixel electrode is electrically connected to the transistor5017. The first pixel electrode and the second pixel electrode areseparated. A shape of the first pixel electrode and the second pixelelectrode is not especially limited. For example, the first pixelelectrode may be a V-like.

A gate electrode of the transistor 5016 is electrically connected to thegate wiring 5012, and a gate electrode of the transistor 5017 iselectrically connected to the gate wiring 5013. When different gatesignals are supplied to the gate wiring 5012 and the gate wiring 5013,operation timings of the transistor 5016 and the transistor 5017 can bevaried. As a result, alignment of liquid crystals can be controlled.

Furthermore, a capacitor may be formed using a capacitor wiring 5010, agate insulator functioning as a dielectric, and a capacitor electrodeelectrically connected to the first pixel electrode or the second pixelelectrode.

The multi-domain pixel includes a first liquid crystal element 5018 anda second liquid crystal element 5019. The first liquid crystal element5018 includes the first pixel electrode, a counter electrode, and aliquid crystal layer therebetween. The second liquid crystal element5019 includes the second pixel electrode, a counter electrode, and aliquid crystal layer therebetween.

Note that a pixel circuit in the display device of one embodiment of thepresent invention is not limited to that shown in FIG. 24B. For example,a switch, a resistor, a capacitor, a transistor, a sensor, a logiccircuit, or the like may be added to the pixel circuit shown in FIG.24B.

[Organic EL Display Device]

FIG. 24C illustrates another example of a circuit configuration of thepixel. Here, a pixel structure of a display device using an organic ELelement is shown.

In an organic EL element, by application of voltage to a light-emittingelement, electrons are injected from one of a pair of electrodesincluded in the organic EL element and holes are injected from the otherof the pair of electrodes, into a layer containing a light-emittingorganic compound; thus, current flows. The electrons and holes arerecombined, and thus, the light-emitting organic compound is excited.The light-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

FIG. 24C illustrates an example of a pixel circuit. Here, one pixelincludes two n-channel transistors. Note that the transistor 490 or thelike can be used as the n-channel transistor. Furthermore, digital timegrayscale driving can be employed for the pixel circuit.

The configuration of the applicable pixel circuit and operation of apixel employing digital time grayscale driving will be described.

A pixel 5020 includes a switching transistor 5021, a driver transistor5022, a light-emitting element 5024, and a capacitor 5023. A gateelectrode of the switching transistor 5021 is connected to a scan line5026, a first electrode (one of a source electrode and a drainelectrode) of the switching transistor 5021 is connected to a signalline 5025, and a second electrode (the other of the source electrode andthe drain electrode) of the switching transistor 5021 is connected to agate electrode of the driver transistor 5022. The gate electrode of thedriver transistor 5022 is connected to a power supply line 5027 throughthe capacitor 5023, a first electrode of the driver transistor 5022 isconnected to the power supply line 5027, and a second electrode of thedriver transistor 5022 is connected to a first electrode (a pixelelectrode) of the light-emitting element 5024. A second electrode of thelight-emitting element 5024 corresponds to a common electrode 5028. Thecommon electrode 5028 is electrically connected to a common potentialline provided over the same substrate.

As each of the switching transistor 5021 and the driver transistor 5022,the transistor 490 or the like can be used as appropriate. In thismanner, an organic EL display device having high display quality and/orhigh reliability can be provided.

The potential of the second electrode (the common electrode 5028) of thelight-emitting element 5024 is set to be a low power supply potential.Note that the low power supply potential is lower than a high powersupply potential supplied to the power supply line 5027. For example,the low power supply potential can be GND, 0 V, or the like. The highpower supply potential and the low power supply potential are set to behigher than or equal to the forward threshold voltage of thelight-emitting element 5024, and the difference between the potentialsis applied to the light-emitting element 5024, whereby current issupplied to the light-emitting element 5024, leading to light emission.The forward voltage of the light-emitting element 5024 refers to avoltage at which a desired luminance is obtained, and includes at leastforward threshold voltage.

Note that gate capacitance of the driver transistor 5022 may be used asa substitute for the capacitor 5023 in some cases, so that the capacitor5023 can be omitted. The gate capacitance of the driver transistor 5022may be formed between the channel formation region and the gateelectrode.

Next, a signal input to the driver transistor 5022 is described. In thecase of a voltage-input voltage driving method, a video signal forturning on or off the driver transistor 5022 is input to the drivertransistor 5022. In order for the driver transistor 5022 to operate in alinear region, voltage higher than the voltage of the power supply line5027 is applied to the gate electrode of the driver transistor 5022.Note that voltage higher than or equal to voltage which is the sum ofpower supply line voltage and the threshold voltage V_(th) of the drivertransistor 5022 is applied to the signal line 5025.

In the case of performing analog grayscale driving, a voltage higherthan or equal to a voltage which is the sum of the forward voltage ofthe light-emitting element 5024 and the threshold voltage V_(th) of thedriver transistor 5022 is applied to the gate electrode of the drivertransistor 5022. A video signal by which the driver transistor 5022 isoperated in a saturation region is input, so that current is supplied tothe light-emitting element 5024. In order for the driver transistor 5022to operate in a saturation region, the potential of the power supplyline 5027 is set higher than the gate potential of the driver transistor5022. When an analog video signal is used, it is possible to supplycurrent to the light-emitting element 5024 in accordance with the videosignal and perform analog grayscale driving.

Note that in the display device of one embodiment of the presentinvention, a pixel configuration is not limited to that shown in FIG.24C. For example, a switch, a resistor, a capacitor, a sensor, atransistor, a logic circuit, or the like may be added to the pixelcircuit shown in FIG. 24C.

In the case where the transistor 490 or the like is used for the circuitshown in FIGS. 24A to 24C, the source electrode (the first electrode) iselectrically connected to the low potential side and the drain electrode(the second electrode) is electrically connected to the high potentialside. Furthermore, the potential of the first gate electrode may becontrolled by a control circuit or the like and the potential describedabove as an example, e.g., a potential lower than the potential appliedto the source electrode, may be input to the second gate electrode.

For example, in this specification and the like, a display element, adisplay device which is a device including a display element, alight-emitting element, and a light-emitting device which is a deviceincluding a light-emitting element can employ a variety of modes or caninclude a variety of elements. A display element, a display device, alight-emitting element, or a light-emitting device includes at least oneof an EL element (e.g., an EL element including organic and inorganicmaterials, an organic EL element, or an inorganic EL element), an LED(e.g., a white LED, a red LED, a green LED, or a blue LED), a transistor(a transistor which emits light depending on current), an electronemitter, a liquid crystal element, electronic ink, an electrophoreticelement, a grating light valve (GLV), a plasma display panel (PDP), amicro electro mechanical system (MEMS), a digital micromirror device(DMD), a digital micro shutter (DMS), an interferometric modulatordisplay (IMOD) element, an electrowetting element, a piezoelectricceramic display, and a display element including a carbon nanotube.Other than the above, display media whose contrast, luminance,reflectivity, transmittance, or the like is changed by electrical ormagnetic effect may be included. Note that examples of a display devicehaving an EL element include an EL display. Examples of a display devicehaving an electron emitter include a field emission display (FED) and anSED-type flat panel display (SED: surface-conduction electron-emitterdisplay). Examples of a display device having a liquid crystal elementinclude a liquid crystal display (e.g., a transmissive liquid crystaldisplay, a transflective liquid crystal display, a reflective liquidcrystal display, a direct-view liquid crystal display, or a projectionliquid crystal display). Examples of a display device having electronicink or an electrophoretic element include electronic paper.

A colored layer (also referred to as a color filter) may be used inorder to obtain a full-color display device in which white light (W) fora backlight (e.g., an organic EL element, an inorganic EL element, anLED, or a fluorescent lamp) is used. As the colored layer, red (R),green (G), blue (B), yellow (Y), or the like may be combined asappropriate, for example. With the use of the colored layer, highercolor reproducibility can be obtained than in the case without thecolored layer. In this case, by providing a region with the coloredlayer and a region without the colored layer, white light in the regionwithout the colored layer may be directly utilized for display. Bypartly providing the region without the colored layer, a decrease inluminance due to the colored layer can be suppressed, and 20% to 30% ofpower consumption can be reduced in some cases when an image isdisplayed brightly. Note that in the case where full-color display isperformed using a self-luminous element such as an organic EL element oran inorganic EL element, elements may emit light of their respectivecolors R, G, B, Y, and W. By using a self-luminous element, powerconsumption can be further reduced as compared to the case of using thecolored layer in some cases.

<Module>

A display module using a semiconductor device of one embodiment of thepresent invention is described below with reference to FIG. 25.

In a display module 8000 in FIG. 25, a touch panel 8004 connected to anFPC 8003, a cell 8006 connected to an FPC 8005, a backlight unit 8007, aframe 8009, a printed board 8010, and a battery 8011 are providedbetween an upper cover 8001 and a lower cover 8002. Note that thebacklight unit 8007, the battery 8011, the touch panel 8004, and thelike are not provided in some cases.

The semiconductor device of one embodiment of the present invention canbe used for the cell 8006, for example.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the cell 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and may be formed to overlap with the cell 8006. A countersubstrate (sealing substrate) of the cell 8006 can have a touch panelfunction. A photosensor may be provided in each pixel of the cell 8006so that an optical touch panel is obtained. An electrode for a touchsensor may be provided in each pixel of the cell 8006 so that acapacitive touch panel is obtained. An electrode for a touch sensor maybe provided in each pixel of the cell 8006 so that a capacitive touchpanel is obtained.

The backlight unit 8007 includes a light source 8008. The light source8008 may be provided at an end portion of the backlight unit 8007 and alight diffusing plate may be used.

The frame 8009 may protect the cell 8006 and also function as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 may function asa radiator plate.

The printed board 8010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or a power source using the battery8011 provided separately may be used. The battery 8011 can be omitted inthe case of using a commercial power source.

The display module 8000 can be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

<Electronic Device>

The semiconductor device of one embodiment of the present invention canbe used for display devices, personal computers, or image reproducingdevices provided with recording media (typically, devices whichreproduce the content of recording media such as digital versatile discs(DVDs) and have displays for displaying the reproduced images). Otherexamples of electronic devices that can be equipped with thesemiconductor device of one embodiment of the present invention aremobile phones, game machines including portable game consoles, portabledata appliances, e-book readers, cameras such as video cameras anddigital still cameras, goggle-type displays (head mounted displays),navigation systems, audio reproducing devices (e.g., car audio systemsand digital audio players), copiers, facsimiles, printers, multifunctionprinters, automated teller machines (ATM), and vending machines. FIGS.26A to 26F illustrate specific examples of these electronic devices.

FIG. 26A illustrates a portable game console including a housing 901, ahousing 902, a display portion 903, a display portion 904, a microphone905, a speaker 906, an operation key 907, a stylus 908, and the like.Although the portable game console in FIG. 26A has the two displayportions 903 and 904, the number of display portions included in aportable game console is not limited to this.

FIG. 26B illustrates a portable data terminal including a first housing911, a second housing 912, a first display portion 913, a second displayportion 914, a joint 915, an operation key 916, and the like. The firstdisplay portion 913 is provided in the first housing 911, and the seconddisplay portion 914 is provided in the second housing 912. The firsthousing 911 and the second housing 912 are connected to each other withthe joint 915, and the angle between the first housing 911 and thesecond housing 912 can be changed with the joint 915. An image on thefirst display portion 913 may be switched depending on the angle betweenthe first housing 911 and the second housing 912 at the joint 915. Adisplay device with a position input function may be used as at leastone of the first display portion 913 and the second display portion 914.Note that the position input function can be added by providing a touchpanel in a display device. Alternatively, the position input functioncan be added by provision of a photoelectric conversion element called aphotosensor in a pixel portion of a display device.

FIG. 26C illustrates a laptop personal computer, which includes ahousing 921, a display portion 922, a keyboard 923, a pointing device924, and the like.

FIG. 26D illustrates the electric refrigerator-freezer including ahousing 931, a door for a refrigerator 932, a door for a freezer 933,and the like.

FIG. 26E illustrates a video camera, which includes a first housing 941,a second housing 942, a display portion 943, operation keys 944, a lens945, a joint 946, and the like. The operation keys 944 and the lens 945are provided for the first housing 941, and the display portion 943 isprovided for the second housing 942. The first housing 941 and thesecond housing 942 are connected to each other with the joint 946, andthe angle between the first housing 941 and the second housing 942 canbe changed with the joint 946. Images displayed on the display portion943 may be switched in accordance with the angle at the joint 946between the first housing 941 and the second housing 942.

FIG. 26F illustrates an ordinary vehicle including a car body 951,wheels 952, a dashboard 953, lights 954, and the like.

<Electronic Device with Curved Display Region or Curved Light-EmittingRegion>

Electronic devices with a curved display region or a curvedlight-emitting region, which are embodiments of the present invention,are described below with reference to FIGS. 27A1, 27A2, 27A3, 27B1,27B2, 27C1, and 27C2. Here, information devices, in particular, portableinformation devices (portable devices) are described as examples of theelectronic devices. The portable information devices include, forexample, mobile phone devices (e.g., phablets and smartphones) andtablet terminals (slate PCs).

FIG. 27A1 is a perspective view illustrating an external shape of aportable device 1300A. FIG. 27A2 is a top view illustrating the portabledevice 1300A. FIG. 27A3 illustrates a usage state of the portable device1300A.

FIGS. 27B1 and 27B2 are perspective views illustrating the outward formof a portable device 1300B.

FIGS. 27C1 and 27C2 are perspective views illustrating the outward formof a portable device 1300C.

<Portable Device>

The portable device 1300A has one or more functions of a telephone,email creating and reading, a notebook, information browsing, and thelike.

A display portion of the portable device 1300A is provided along pluralsurfaces. For example, the display portion may be provided by placing aflexible display device along the inside of a housing. Thus, text data,image data, or the like can be displayed on a first region 1311 and/or asecond region 1312.

For example, images used for three operations can be displayed on thefirst region 1311 (see FIG. 27A1). Furthermore, text data and the likecan be displayed on the second region 1312 as indicated by dashedrectangles in the drawing (see FIG. 27A2).

In the case where the second region 1312 is on the upper portion of theportable device 1300A, a user can easily see text data or image datadisplayed on the second region 1312 of the portable device 1300A whilethe portable device 1300A is placed in a breast pocket of the user'sclothes (see FIG. 27A3). For example, the user can see the phone number,name, and the like of the caller of an incoming call, from above theportable device 1300A.

The portable device 1300A may include an input device or the likebetween the display device and the housing, in the display device, orover the housing. As the input device, for example, a touch sensor, alight sensor, or an ultrasonic sensor may be used. In the case where theinput device is provided between the display device and the housing orover the housing, a touch panel may be, for example, a matrix switchtype, a resistive type, an ultrasonic surface acoustic wave type, aninfrared type, electromagnetic induction type, or an electrostaticcapacitance type. In the case where the input device is provided in thedisplay device, an in-cell sensor, an on-cell sensor, or the like may beused.

Note that the portable device 1300A can be provided with a vibrationsensor or the like and a memory device that stores a program forshifting a mode into an incoming call rejection mode based on vibrationsensed by the vibration sensor or the like. Thus, the user can shift themode into the incoming call rejection mode by tapping the portabledevice 1300A over his/her clothes to apply vibration.

The portable device 1300B includes a display portion including the firstregion 1311 and the second region 1312 and a housing 1310 that supportsthe display portion.

The housing 1310 has a plurality of bend portions, and the longest bendportion in the housing 1310 is between the first region 1311 and thesecond region 1312.

The portable device 1300B can be used with the second region 1312provided along the longest bend portion facing sideward.

The portable device 1300C includes a display portion including the firstregion 1311 and the second region 1312 and the housing 1310 thatsupports the display portion.

The housing 1310 has a plurality of bend portions, and the secondlongest bend portion in the housing 1310 is between the first region1311 and the second region 1312.

The portable device 1300C can be used with the second region 1312 facingupward.

Note that contents that are not specified in any drawing or text in thespecification can be excluded from one embodiment of the invention.Alternatively, when the range of a value that is defined by the maximumand minimum values is described, part of the range is appropriatelynarrowed or part of the range is removed, whereby one embodiment of theinvention excluding part of the range can be constructed. In thismanner, it is possible to specify the technical scope of one embodimentof the present invention so that a conventional technology is excluded,for example.

As another specific example, a description “a film is an insulator” isgiven to describe properties of a material. In that case, for example,it can be specified that the case where the insulator is an organicinsulator is excluded from one embodiment of the invention. For example,it can be specified that the case where the insulator is an inorganicinsulator is excluded from one embodiment of the invention. For example,it can be specified that the case where the film is a conductor isexcluded from one embodiment of the invention. For example, it can bespecified that the case where the film is a semiconductor is excludedfrom one embodiment of the invention.

As another specific example, the description of a stacked structure, “afilm is provided between an A film and a B film” is given. In that case,for example, it can be specified that the case where the film is astacked film of four or more layers is excluded from the invention. Forexample, it can be specified that the case where a conductor is providedbetween the A film and the film is excluded from the invention.

Note that various people can implement one embodiment of the inventiondescribed in this specification and the like. However, different peoplemay be involved in the implementation of the invention. For example, inthe case of a transmission/reception system, the following case ispossible: Company A manufactures and sells transmitting devices, andCompany B manufactures and sells receiving devices. As another example,in the case of a light-emitting device including a transistor and alight-emitting element, the following case is possible: Company Amanufactures and sells semiconductor devices including transistors, andCompany B purchases the semiconductor devices, provides light-emittingelements for the semiconductor devices, and completes light-emittingdevices.

In such a case, one embodiment of the invention can be constituted sothat a patent infringement can be claimed against each of Company A andCompany B. In other words, one embodiment of the invention can beconstituted so that only Company A implements the embodiment, andanother embodiment of the invention can be constituted so that onlyCompany B implements the embodiment. One embodiment of the inventionwith which a patent infringement suit can be filed against Company A orCompany B is clear and can be regarded as being disclosed in thisspecification or the like. For example, in the case of atransmission/reception system, even when this specification or the likedoes not include a description of the case where a transmitting deviceis used alone or the case where a receiving device is used alone, oneembodiment of the invention can be constituted by only the transmittingdevice and another embodiment of the invention can be constituted byonly the receiving device. Those embodiments of the invention are clearand can be regarded as being disclosed in this specification or thelike. Another example is as follows: in the case of a light-emittingdevice including a transistor and a light-emitting element, even whenthis specification or the like does not include a description of thecase where a semiconductor device including the transistor is used aloneor the case where a light-emitting device including the light-emittingelement is used alone, one embodiment of the invention can beconstituted by only the semiconductor device including the transistorand another embodiment of the invention can be constituted by only thelight-emitting device including the light-emitting element. Thoseembodiments of the invention are clear and can be regarded as beingdisclosed in this specification or the like.

Note that in this specification and the like, it might be possible forthose skilled in the art to constitute one embodiment of the inventioneven when portions to which all the terminals of an active element(e.g., a transistor or a diode), a passive element (e.g., a capacitor ora resistor), or the like are connected are not specified. In otherwords, one embodiment of the invention can be clear even when connectionportions are not specified. Further, in the case where a connectionportion is disclosed in this specification and the like, it can bedetermined that one embodiment of the invention in which a connectionportion is not specified is disclosed in this specification and thelike, in some cases. In particular, in the case where the number ofportions to which the terminal is connected might be plural, it is notnecessary to specify the portions to which the terminal is connected.Therefore, it might be possible to constitute one embodiment of theinvention by specifying only portions to which some of terminals of anactive element (e.g., a transistor or a diode), a passive element (e.g.,a capacitor or a resistor), or the like are connected.

Note that in this specification and the like, it might be possible forthose skilled in the art to specify the invention when at least theconnection portion of a circuit is specified. Alternatively, it might bepossible for those skilled in the art to specify the invention when atleast a function of a circuit is specified. In other words, when afunction of a circuit is specified, one embodiment of the invention canbe clear. Furthermore, it can be determined that one embodiment of theinvention whose function is specified is disclosed in this specificationand the like. Therefore, when a connection portion of a circuit isspecified, the circuit is disclosed as one embodiment of the inventioneven when a function is not specified, and one embodiment of theinvention can be constituted. Alternatively, when a function of acircuit is specified, the circuit is disclosed as one embodiment of theinvention even when a connection portion is not specified, and oneembodiment of the invention can be constituted.

Note that in this specification and the like, a content described in atleast a diagram (which may be part of the diagram) is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. Therefore, when a certain content is described in adiagram, the content is disclosed as one embodiment of the inventioneven when the content is not described with a text, and one embodimentof the invention can be constituted. In a similar manner, part of adiagram, which is taken out from the diagram, is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. The embodiment of the invention is clear.

This application is based on Japanese Patent Application serial no.2014-055157 filed with Japan Patent Office on Mar. 18, 2014, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a firsttransistor, the first transistor comprising: a first semiconductor; afirst insulator; and a first conductor; a second insulator over thefirst transistor; a capacitor over the second insulator, the capacitorcomprising: a second conductor; a third conductor; and a thirdinsulator; a fourth insulator over the capacitor; and a secondtransistor over the fourth insulator, the second transistor comprising:a second semiconductor; a fifth insulator; and a fourth conductor,wherein a part of the first conductor overlaps with the firstsemiconductor with the first insulator interposed therebetween, whereina part of the fourth conductor overlaps with the second semiconductorwith the fifth insulator interposed therebetween, wherein a part of thesecond conductor faces the third conductor with the third insulatorinterposed therebetween, wherein the second conductor is electricallyconnected to the first conductor through a first opening in the secondinsulator, and wherein the second conductor is electrically connected tothe second semiconductor through a second opening in the fourthinsulator.
 2. The semiconductor device according to claim 1, wherein thethird insulator comprises an element included in the second conductor.3. The semiconductor device according to claim 1, wherein the firstsemiconductor comprises silicon.
 4. The semiconductor device accordingto claim 1, wherein the second semiconductor comprises indium.
 5. Thesemiconductor device according to claim 1, wherein the firstsemiconductor is a part of a semiconductor substrate.
 6. Thesemiconductor device according to claim 1, wherein the firstsemiconductor has a projection provided on a semiconductor substrate. 7.A semiconductor device comprising: a first transistor, the firsttransistor comprising: a first semiconductor; a first insulator; and afirst conductor; a second insulator over the first transistor; a secondtransistor over the second insulator, the second transistor comprising:a second semiconductor; a third insulator; and a second conductor; afourth insulator over the second transistor; and a capacitor over thefourth insulator, the capacitor comprising: a third conductor; a fourthconductor; and a fifth insulator, wherein a part of the first conductoroverlaps with the first semiconductor with the first insulatorinterposed therebetween, wherein a part of the second conductor overlapswith the second semiconductor with the third insulator interposedtherebetween, wherein a part of the third conductor faces the fourthconductor with the fifth insulator interposed therebetween, wherein thethird conductor is electrically connected to the first conductor througha first opening in the second insulator and a second opening in thefourth insulator, and wherein the third conductor is electricallyconnected to the second semiconductor through the second opening in thefourth insulator.
 8. The semiconductor device according to claim 7,wherein the fifth insulator comprises an element included in the fourthconductor.
 9. The semiconductor device according to claim 7, wherein thefirst semiconductor comprises silicon.
 10. The semiconductor deviceaccording to claim 7, wherein the second semiconductor comprises indium.11. The semiconductor device according to claim 7, wherein the firstsemiconductor is a part of a semiconductor substrate.
 12. Thesemiconductor device according to claim 7, wherein the firstsemiconductor has a projection provided on a semiconductor substrate.